The bug

Mobs with multiple states have their hitboxs shifted when they spawn (except for the first state).
See examples for more infos.

How to reproduce

1. Activate hitboxes with F3+B
2. Spawn a mob (like a Magma Cube or a Slime or a Pufferfish) and look at the hixbox of the mob

Examples

Here are examples in videos:
• Magma Cube: Magma Cube Hitbox Problem (1).mp4 and Magma Cube Hitbox Problem (2).mp4
• Pufferfish: Pufferfish Hitbox Problem.mp4
• Slime: Slime Hitbox Problem.mp4

Additional information

This bug occurs both when the mob appears with an spawn egg or a command (/summon).