Steps to reproduce:
Find or create a pool of water that's at least 2 blocks deep;
Walk into it and watch the camera while you're sinking.
Expected results:
The camera descends with you properly.
Observed results:
The camera pauses at the water surface for a bit and then continues descending after you're already fully submerged.
Notes: This occurs with lava as well, but the fire overlay makes it difficult to reproduce. As of 1.20.40.21 Preview, this issue also causes a mismatch between the player's vision and the newly introduced sound you hear when getting submerged.
This feature isn't present in any change logs, but this is to stop the water texture from looking 2-dimensional when a player's screen passes the boundary. This is typically used in games to make the transition between above-water and in-water more realistic.