Description:

When a mob is standing still, its yaw rotation does not update according to what it is looking at, even though the pitch rotation does update. 

If you use /data to see the rotation of a mob, it does not sync up with the direction the mob is looking at when you have hitboxes turned on.

How to reproduce:

1. Place a block high up in the sky
2. Summon for example a pig
3. use /data get entity @e[type=!player,sort=nearest,limit=1] Rotation
4. Look at Yaw and pitch values
5. Get a carrot 
6. Get the mob to rotate away from its initial rotation
7. use again /data get entity @e[type=!player,sort=nearest,limit=1] Rotation
8. Yaw does not update, but pitch does if you fly up/down around the pig

This bug from what i tested affects all sorts of entities from pigs, zombified piglins, pillagers, polar bears, llamas, etc.

Both screenshots show the /data command which says that the pig is looking in the same direction even though in one case its looking east and in the other south. But pitch is correctly shown.

Link to video showcase of the bug: https://youtu.be/A4XThpGzkfc