diff --git a/petab/core.py b/petab/core.py
index 05deb16..0e7b7da 100644
--- a/petab/core.py
+++ b/petab/core.py
@@ -71,7 +71,7 @@ def write_simulation_df(df: pd.DataFrame, filename: Union[str, Path]) -> None:
 
 
 def get_visualization_df(
-        visualization_file: Union[str, Path, pd.DataFrame, None]
+    visualization_file: Union[str, Path, pd.DataFrame, None]
 ) -> Union[pd.DataFrame, None]:
     """Read PEtab visualization table
 
@@ -254,7 +254,7 @@ def flatten_timepoint_specific_output_overrides(
 
     Arguments:
         petab_problem:
-            PEtab problem to work on
+            PEtab problem to work on. Modified in place.
     """
     new_measurement_dfs = []
     new_observable_dfs = []
@@ -277,22 +277,21 @@ def flatten_timepoint_specific_output_overrides(
         for field, hyperparameter_type, target in [
             (NOISE_PARAMETERS, "noiseParameter", NOISE_FORMULA),
             (OBSERVABLE_PARAMETERS, "observableParameter", OBSERVABLE_FORMULA),
+            (OBSERVABLE_PARAMETERS, "observableParameter", NOISE_FORMULA),
         ]:
-            if field in measurements:
-                hyperparameter_replacement_id = (
-                    get_hyperparameter_replacement_id(
-                        hyperparameter_type=hyperparameter_type,
-                        observable_replacement_id=observable_replacement_id,
-                    )
-                )
-                hyperparameter_id = mappings[field][
-                    hyperparameter_replacement_id
-                ]
-                observable[target] = re.sub(
-                    hyperparameter_id,
-                    hyperparameter_replacement_id,
-                    observable[target],
-                )
+            if field not in measurements:
+                continue
+
+            hyperparameter_replacement_id = get_hyperparameter_replacement_id(
+                hyperparameter_type=hyperparameter_type,
+                observable_replacement_id=observable_replacement_id,
+            )
+            hyperparameter_id = mappings[field][hyperparameter_replacement_id]
+            observable[target] = re.sub(
+                hyperparameter_id,
+                hyperparameter_replacement_id,
+                observable[target],
+            )
 
         measurements[OBSERVABLE_ID] = observable_replacement_id
         new_measurement_dfs.append(measurements)
@@ -306,7 +305,7 @@ def flatten_timepoint_specific_output_overrides(
 def unflatten_simulation_df(
     simulation_df: pd.DataFrame,
     petab_problem: "petab.problem.Problem",
-) -> None:
+) -> pd.DataFrame:
     """Unflatten simulations from a flattened PEtab problem.
 
     A flattened PEtab problem is the output of applying

diff --git a/tests/test_petab.py b/tests/test_petab.py
index 89053fb..b836880 100644
--- a/tests/test_petab.py
+++ b/tests/test_petab.py
@@ -353,36 +353,32 @@ def test_flatten_timepoint_specific_output_overrides():
             OBSERVABLE_FORMULA: [
                 "observableParameter1_obs1 + observableParameter2_obs1"
             ],
-            NOISE_FORMULA: ["noiseParameter1_obs1"],
+            NOISE_FORMULA: [
+                "(observableParameter1_obs1 + observableParameter2_obs1) * noiseParameter1_obs1"
+            ],
         }
     )
     observable_df.set_index(OBSERVABLE_ID, inplace=True)
 
+    # new observable IDs (obs${i_obs}_${i_obsParOverride}_${i_noiseParOverride}_${i_condition})
+    obs1_1_1_1 = "obs1__obsParOverride1_1_0__noiseParOverride1__condition1"
+    obs1_2_1_1 = "obs1__obsParOverride2_1_0__noiseParOverride1__condition1"
+    obs1_2_2_1 = "obs1__obsParOverride2_1_0__noiseParOverride2__condition1"
     observable_df_expected = pd.DataFrame(
         data={
-            OBSERVABLE_ID: [
-                "obs1__obsParOverride1_1_0__noiseParOverride1__condition1",
-                "obs1__obsParOverride2_1_0__noiseParOverride1__condition1",
-                "obs1__obsParOverride2_1_0__noiseParOverride2__condition1",
-            ],
+            OBSERVABLE_ID: [obs1_1_1_1, obs1_2_1_1, obs1_2_2_1],
             OBSERVABLE_FORMULA: [
-                "observableParameter1_obs1__obsParOverride1_1_0__"
-                "noiseParOverride1__condition1 + observableParameter2_obs1"
-                "__obsParOverride1_1_0__noiseParOverride1__condition1",
-                "observableParameter1_obs1__obsParOverride2_1_0__noiseParOverride1"
-                "__condition1 + observableParameter2_obs1__obsParOverride2_1_0"
-                "__noiseParOverride1__condition1",
-                "observableParameter1_obs1__obsParOverride2_1_0"
-                "__noiseParOverride2__condition1 + observableParameter2_obs1__"
-                "obsParOverride2_1_0__noiseParOverride2__condition1",
+                f"observableParameter1_{obs1_1_1_1} + observableParameter2_{obs1_1_1_1}",
+                f"observableParameter1_{obs1_2_1_1} + observableParameter2_{obs1_2_1_1}",
+                f"observableParameter1_{obs1_2_2_1} + observableParameter2_{obs1_2_2_1}",
             ],
             NOISE_FORMULA: [
-                "noiseParameter1_obs1__obsParOverride1_1_0__"
-                "noiseParOverride1__condition1",
-                "noiseParameter1_obs1__obsParOverride2_1_0__"
-                "noiseParOverride1__condition1",
-                "noiseParameter1_obs1__obsParOverride2_1_0__"
-                "noiseParOverride2__condition1",
+                f"(observableParameter1_{obs1_1_1_1} + observableParameter2_{obs1_1_1_1})"
+                f" * noiseParameter1_{obs1_1_1_1}",
+                f"(observableParameter1_{obs1_2_1_1} + observableParameter2_{obs1_2_1_1})"
+                f" * noiseParameter1_{obs1_2_1_1}",
+                f"(observableParameter1_{obs1_2_2_1} + observableParameter2_{obs1_2_2_1})"
+                f" * noiseParameter1_{obs1_2_2_1}",
             ],
         }
     )
@@ -418,12 +414,7 @@ def test_flatten_timepoint_specific_output_overrides():
 
     measurement_df_expected = pd.DataFrame(
         data={
-            OBSERVABLE_ID: [
-                "obs1__obsParOverride1_1_0__noiseParOverride1__condition1",
-                "obs1__obsParOverride2_1_0__noiseParOverride1__condition1",
-                "obs1__obsParOverride2_1_0__noiseParOverride2__condition1",
-                "obs1__obsParOverride2_1_0__noiseParOverride2__condition1",
-            ],
+            OBSERVABLE_ID: [obs1_1_1_1, obs1_2_1_1, obs1_2_2_1, obs1_2_2_1],
             SIMULATION_CONDITION_ID: [
                 "condition1",
                 "condition1",
@@ -472,8 +463,12 @@ def test_flatten_timepoint_specific_output_overrides():
         is False
     )
 
-    assert problem.observable_df.equals(observable_df_expected) is True
-    assert problem.measurement_df.equals(measurement_df_expected) is True
+    pd.testing.assert_frame_equal(
+        problem.observable_df, observable_df_expected
+    )
+    pd.testing.assert_frame_equal(
+        problem.measurement_df, measurement_df_expected
+    )
 
     assert petab.lint_problem(problem) is False
 
@@ -591,8 +586,12 @@ def test_flatten_timepoint_specific_output_overrides_special_cases():
         is False
     )
 
-    assert problem.observable_df.equals(observable_df_expected) is True
-    assert problem.measurement_df.equals(measurement_df_expected) is True
+    pd.testing.assert_frame_equal(
+        problem.observable_df, observable_df_expected
+    )
+    pd.testing.assert_frame_equal(
+        problem.measurement_df, measurement_df_expected
+    )
 
     assert petab.lint_problem(problem) is False
 
@@ -842,13 +841,14 @@ def test_get_required_parameters_for_parameter_table(petab_problem):
     # as part of the proportional error model.
     assert "observableParameter1_obs1" in noise_placeholders
 
-    required_parameters_for_parameter_table = \
+    required_parameters_for_parameter_table = (
         petab.parameters.get_required_parameters_for_parameter_table(
             model=petab_problem.model,
             condition_df=petab_problem.condition_df,
             observable_df=petab_problem.observable_df,
             measurement_df=petab_problem.measurement_df,
         )
+    )
     # The observable parameter is correctly recognized as a placeholder,
     # i.e. does not need to be in the parameter table.
     assert (

`flatten_timepoint_specific_output_overrides` does not support observableParameter overrides as placeholders in noise formulae
`flatten_timepoint_specific_output_overrides`  does not support `observableParameter` placeholders in noise formulae.

Related to https://github.com/PEtab-dev/libpetab-python/pull/231
tests/test_petab.py::test_flatten_timepoint_specific_output_overrides
tests/test_petab.py::test_split_parameter_replacement_list tests/test_petab.py::test_get_measurement_parameter_ids tests/test_petab.py::test_serialization tests/test_petab.py::test_get_priors_from_df tests/test_petab.py::test_startpoint_sampling tests/test_petab.py::test_startpoint_sampling_dict tests/test_petab.py::test_create_parameter_df tests/test_petab.py::test_flatten_timepoint_specific_output_overrides_special_cases tests/test_petab.py::test_concat_measurements tests/test_petab.py::test_concat_condition_df tests/test_petab.py::test_get_observable_ids tests/test_petab.py::test_parameter_properties tests/test_petab.py::test_to_float_if_float tests/test_petab.py::test_to_files tests/test_petab.py::test_load_remote tests/test_petab.py::test_problem_from_yaml_v1_empty tests/test_petab.py::test_problem_from_yaml_v1_multiple_files tests/test_petab.py::test_get_required_parameters_for_parameter_table