The bug

The player goes into swimming mode when he has only one block of top available, even if he is not in water.

How to reproduce

1. Put yourself below pistons
2. Activate the pistons to push the blocks above your head
3. With only one block up, your player will automatically switch to swimming mode, even if he is not in water

Example

Here is an example in video: Player goes into swimming mode when he has only one block of top available even if he is not in water.mp4

Additional context

This bug is due to the correction of MC-90594 and MC-146332 in the 19w14b snapshot.