Oxygen bar filling up underwater after briefly exiting swimming pose

The bug

After briefly exiting the swimming pose, a server-client desync can happen whether the player is in the swimming pose. This makes you able to refill your oxygen meter close to, but not above, the water surface.
This can be reproduced by e.g. killing a fish while swimming in full speed.

How to reproduce

1. Find some water
2. /summon cod
3. start swimming
4. Try to hit the cod without slowing down
5. If you did not leave the swimming pose briefly, try again
6. Otherwise swim close to the surface
   → Your air bar refills