The bug
When a mob wearing leather boots is walking around on top of powdered snow, it will often end up spinning around itself for a period of time. In the same way as mobs do on waterlogged slaps and double layer carpets etc.
Linked issues
relates to
Comments

Can confirm in 20w51a.
Can confirm in 21w03a. This happens quite rarely so it may be difficult to reproduce.
I can confirm that of all mobs i tested, only Skeletons and Stray were able to move around, other mobs only stood still.