Many mobs don't drown in water if they are directly under blocks

Some mobs like sheep or horses do not drown in water if they are trapped floating up against blocks (pigs and foxes, for example, do drown).

Steps to reproduce:

1. Find an ice river/ice ocean or a water-filled cave (or place blocks in water).

2. Summon a sheep or a horse under the ice/cave ceiling/blocks (more mobs may work, requires further testing).

3. The mob now floats up against the ceiling.

4. ❌ It does not drown despite being trapped in water.

Edit: After further testing I found that the bug also affects cows, donkeys, mules, llamas, camels, chickens, ocelots and probably many more types of mobs.