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