When an entity enters into water, its underwater bounding box may disappeared if you press F3+B. Sometimes the underwater part of an entity may disappeared. The same effect occurs when the player is underwater and the entity is on shore. It seems that the sunface of water cuts the bounding box open. The possiability of disappearance may be related to the pitch angle.
I also test old versions, the bug can only be reproduced in 1.16 and higher versions.
Affect entities:
Mobs (Animals, Monstors and so on)
Area Effect Clouds
Items
Experience Orbs
Leash Knots
Paintings
Item Frames
Armor Stands
TNT
Players
Fishing Bobbors
Minecarts
End Crystals
Projectiles
To do tests:
Thunders
Evoker Fangs
Linked issues
duplicates
Attachments
Comments

Same issue I think
Relates to MC-197328