diff --git a/pydantic/main.py b/pydantic/main.py
--- a/pydantic/main.py
+++ b/pydantic/main.py
@@ -666,7 +666,7 @@ def __get_validators__(cls) -> 'CallableGenerator':
     def validate(cls: Type['Model'], value: Any) -> 'Model':
         if isinstance(value, cls):
             if cls.__config__.copy_on_model_validation:
-                return value._copy_and_set_values(value.__dict__, value.__fields_set__, deep=False)
+                return value._copy_and_set_values(value.__dict__, value.__fields_set__, deep=True)
             else:
                 return value
 

diff --git a/tests/test_main.py b/tests/test_main.py
--- a/tests/test_main.py
+++ b/tests/test_main.py
@@ -1561,12 +1561,28 @@ class Config:
 
     assert t.user is not my_user
     assert t.user.hobbies == ['scuba diving']
-    assert t.user.hobbies is my_user.hobbies  # `Config.copy_on_model_validation` only does a shallow copy
+    assert t.user.hobbies is not my_user.hobbies  # `Config.copy_on_model_validation` does a deep copy
     assert t.user._priv == 13
     assert t.user.password.get_secret_value() == 'hashedpassword'
     assert t.dict() == {'id': '1234567890', 'user': {'id': 42, 'hobbies': ['scuba diving']}}
 
 
+def test_validation_deep_copy():
+    """By default, Config.copy_on_model_validation should do a deep copy"""
+
+    class A(BaseModel):
+        name: str
+
+    class B(BaseModel):
+        list_a: List[A]
+
+    a = A(name='a')
+    b = B(list_a=[a])
+    assert b.list_a == [A(name='a')]
+    a.name = 'b'
+    assert b.list_a == [A(name='a')]
+
+
 @pytest.mark.parametrize(
     'kinds',
     [

copy_on_model_validation config doesn't copy List field anymore in v1.9.0
### Checks

* [x] I added a descriptive title to this issue
* [x] I have searched (google, github) for similar issues and couldn't find anything
* [x] I have read and followed [the docs](https://pydantic-docs.helpmanual.io/) and still think this is a bug

<!-- Sorry to sound so draconian, but every second saved replying to issues is time spend improving pydantic :-) -->

# Bug

The output of `python -c "import pydantic.utils; print(pydantic.utils.version_info())"` when I use v1.9.0a1,
```
             pydantic version: 1.9.0a1
            pydantic compiled: True
                 install path: <*my-project-dir*>/venv/lib/python3.9/site-packages/pydantic
               python version: 3.9.9 (main, Nov 16 2021, 03:05:18)  [GCC 7.5.0]
                     platform: Linux-5.4.0-91-generic-x86_64-with-glibc2.27
     optional deps. installed: ['typing-extensions']
```
The output of `python -c "import pydantic.utils; print(pydantic.utils.version_info())"` when I use v1.9.0,
```
             pydantic version: 1.9.0
            pydantic compiled: True
                 install path: <*my-project-dir*>/venv/lib/python3.9/site-packages/pydantic
               python version: 3.9.9 (main, Nov 16 2021, 03:05:18)  [GCC 7.5.0]
                     platform: Linux-5.4.0-91-generic-x86_64-with-glibc2.27
     optional deps. installed: ['typing-extensions']
```

<!-- or if you're using pydantic prior to v1.3, manually include: OS, python version and pydantic version -->

<!-- Please read the [docs](https://pydantic-docs.helpmanual.io/) and search through issues to
confirm your bug hasn't already been reported. -->

<!-- Where possible please include a self-contained code snippet describing your bug: -->

Looks like there is accidental behaviour change from v1.9.0a1 to v1.9.0 regarding the `copy_on_model_validation` config.

```py
from pydantic import BaseModel
from typing import List


class A(BaseModel):
    name: str
    
    class Config:
        copy_on_model_validation = True  # Default behaviour
        

class B(BaseModel):
    list_a: List[A]


a = A(name="a")
b = B(list_a=[a])

print(a)
print(b)
a.name = "b"  # make a change to variable a's field
print(a)
print(b)

```
The output using v1.9.0a1,
```
A(name='a')
B(list_a=[A(name='a')])
A(name='b')
B(list_a=[A(name='a')])
```

The output using v1.9.0 changed to,
```
A(name='a')
B(list_a=[A(name='a')])
A(name='b')
B(list_a=[A(name='b')])
```



tests/test_main.py::test_model_exclude_copy_on_model_validation tests/test_main.py::test_validation_deep_copy
tests/test_main.py::test_success tests/test_main.py::test_ultra_simple_missing tests/test_main.py::test_ultra_simple_failed tests/test_main.py::test_ultra_simple_repr tests/test_main.py::test_default_factory_field tests/test_main.py::test_default_factory_no_type_field tests/test_main.py::test_comparing tests/test_main.py::test_nullable_strings_success tests/test_main.py::test_nullable_strings_fails tests/test_main.py::test_recursion tests/test_main.py::test_recursion_fails tests/test_main.py::test_not_required tests/test_main.py::test_infer_type tests/test_main.py::test_allow_extra tests/test_main.py::test_allow_extra_repr tests/test_main.py::test_forbidden_extra_success tests/test_main.py::test_forbidden_extra_fails tests/test_main.py::test_disallow_mutation tests/test_main.py::test_extra_allowed tests/test_main.py::test_extra_ignored tests/test_main.py::test_set_attr tests/test_main.py::test_set_attr_invalid tests/test_main.py::test_any tests/test_main.py::test_alias tests/test_main.py::test_population_by_field_name tests/test_main.py::test_field_order tests/test_main.py::test_required tests/test_main.py::test_mutability tests/test_main.py::test_immutability[False-False] tests/test_main.py::test_immutability[False-True] tests/test_main.py::test_immutability[True-True] tests/test_main.py::test_not_frozen_are_not_hashable tests/test_main.py::test_with_declared_hash tests/test_main.py::test_frozen_with_hashable_fields_are_hashable tests/test_main.py::test_frozen_with_unhashable_fields_are_not_hashable tests/test_main.py::test_hash_function_give_different_result_for_different_object tests/test_main.py::test_const_validates tests/test_main.py::test_const_uses_default tests/test_main.py::test_const_validates_after_type_validators tests/test_main.py::test_const_with_wrong_value tests/test_main.py::test_const_with_validator tests/test_main.py::test_const_list tests/test_main.py::test_const_list_with_wrong_value tests/test_main.py::test_const_validation_json_serializable tests/test_main.py::test_validating_assignment_pass tests/test_main.py::test_validating_assignment_fail tests/test_main.py::test_validating_assignment_pre_root_validator_fail tests/test_main.py::test_validating_assignment_post_root_validator_fail tests/test_main.py::test_root_validator_many_values_change tests/test_main.py::test_enum_values tests/test_main.py::test_literal_enum_values tests/test_main.py::test_enum_raw tests/test_main.py::test_set_tuple_values tests/test_main.py::test_default_copy tests/test_main.py::test_arbitrary_type_allowed_validation_success tests/test_main.py::test_arbitrary_type_allowed_validation_fails tests/test_main.py::test_arbitrary_types_not_allowed tests/test_main.py::test_type_type_validation_success tests/test_main.py::test_type_type_subclass_validation_success tests/test_main.py::test_type_type_validation_fails_for_instance tests/test_main.py::test_type_type_validation_fails_for_basic_type tests/test_main.py::test_bare_type_type_validation_success tests/test_main.py::test_bare_type_type_validation_fails tests/test_main.py::test_annotation_field_name_shadows_attribute tests/test_main.py::test_value_field_name_shadows_attribute tests/test_main.py::test_class_var tests/test_main.py::test_fields_set tests/test_main.py::test_exclude_unset_dict tests/test_main.py::test_exclude_unset_recursive tests/test_main.py::test_dict_exclude_unset_populated_by_alias tests/test_main.py::test_dict_exclude_unset_populated_by_alias_with_extra tests/test_main.py::test_exclude_defaults tests/test_main.py::test_dir_fields tests/test_main.py::test_dict_with_extra_keys tests/test_main.py::test_root tests/test_main.py::test_root_list tests/test_main.py::test_root_nested tests/test_main.py::test_encode_nested_root tests/test_main.py::test_root_failed tests/test_main.py::test_root_undefined_failed tests/test_main.py::test_parse_root_as_mapping tests/test_main.py::test_parse_obj_non_mapping_root tests/test_main.py::test_parse_obj_nested_root tests/test_main.py::test_untouched_types tests/test_main.py::test_custom_types_fail_without_keep_untouched tests/test_main.py::test_model_iteration tests/test_main.py::test_model_export_nested_list[excluding tests/test_main.py::test_model_export_nested_list[should tests/test_main.py::test_model_export_nested_list[using tests/test_main.py::test_model_export_dict_exclusion[excluding tests/test_main.py::test_model_export_dict_exclusion[using tests/test_main.py::test_model_export_dict_exclusion[exclude tests/test_main.py::test_model_exclude_config_field_merging tests/test_main.py::test_model_export_exclusion_with_fields_and_config[None-expected0-kinds0] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[None-expected0-kinds1] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[None-expected0-kinds2] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[None-expected0-kinds3] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[None-expected0-kinds4] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[None-expected0-kinds5] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[None-expected0-kinds6] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude1-expected1-kinds0] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude1-expected1-kinds1] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude1-expected1-kinds2] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude1-expected1-kinds3] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude1-expected1-kinds4] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude1-expected1-kinds5] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude1-expected1-kinds6] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude2-expected2-kinds0] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude2-expected2-kinds1] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude2-expected2-kinds2] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude2-expected2-kinds3] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude2-expected2-kinds4] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude2-expected2-kinds5] tests/test_main.py::test_model_export_exclusion_with_fields_and_config[exclude2-expected2-kinds6] tests/test_main.py::test_model_export_exclusion_inheritance tests/test_main.py::test_model_export_with_true_instead_of_ellipsis tests/test_main.py::test_model_export_inclusion tests/test_main.py::test_model_export_inclusion_inheritance tests/test_main.py::test_custom_init_subclass_params tests/test_main.py::test_update_forward_refs_does_not_modify_module_dict tests/test_main.py::test_two_defaults tests/test_main.py::test_default_factory tests/test_main.py::test_default_factory_called_once tests/test_main.py::test_default_factory_called_once_2 tests/test_main.py::test_default_factory_validate_children tests/test_main.py::test_default_factory_parse tests/test_main.py::test_none_min_max_items tests/test_main.py::test_reuse_same_field tests/test_main.py::test_base_config_type_hinting tests/test_main.py::test_allow_mutation_field tests/test_main.py::test_repr_field tests/test_main.py::test_inherited_model_field_copy tests/test_main.py::test_inherited_model_field_untouched tests/test_main.py::test_mapping_retains_type_subclass tests/test_main.py::test_mapping_retains_type_defaultdict tests/test_main.py::test_mapping_retains_type_fallback_error tests/test_main.py::test_typing_coercion_dict tests/test_main.py::test_typing_non_coercion_of_dict_subclasses tests/test_main.py::test_typing_coercion_defaultdict tests/test_main.py::test_typing_coercion_counter tests/test_main.py::test_typing_counter_value_validation tests/test_main.py::test_class_kwargs_config tests/test_main.py::test_class_kwargs_config_json_encoders tests/test_main.py::test_class_kwargs_config_and_attr_conflict tests/test_main.py::test_class_kwargs_custom_config