The bug:
If you have hitboxes shown (F3+B), your player's model will update one frame after you start sneaking. The video below can show you what this means. This is also noticeable without hitboxes shown if you are using a lead to connect your body to an entity.

Reproduce:
1. Turn on hitboxes with F3+B (or use a lead on an entity to connect your body to the entity)
2. Enter 3rd person mode to see your player's hitbox (or the lead connecting your body to the entity)
3. Sneak
-> The model updated one frame late

Note:
This is a very trivial bug.