Minecraft wiki entry for Development_versions, associated text: MC-239854 – Nether and caves world is missing bedrock.
MC-239858 – Nether decorations can be generated on the original bedrock layer.
MC-239866 – Terrain shaper in noise_settings is not used.
MC-239876 – Maps appear gray when opened in old terrain after conversion.
MC-239882 – Beacons deactivate when upgrading worlds.
MC-239902 – Weather effects appear in caves and structures in 1.17.1 worlds loaded in 21w43a.