The bug
When skeletons, drowned or zombies are underwater, they sometimes start to spin fast. Usually a few seconds after they first enter the water, and every few seconds after that.
Related issues
is duplicated by
relates to
Comments

Can confirm in 20w51a. I did also experience this with iron golems.
Can confirm in 21w03a.
Can confirm in 21w05b.

I confirmed it with 1.16.5 and 1.17 Pre-release 1

Confirm in 1.17 Pre-release 2

I confirmed it with 1.17 Pre-release 3

I confirmed it with 1.17 Release Candidate 1

Can confirm in 1.17 (reproduced while testing MC-228448).

Can confirm in 1.18 Pre-release 1.
Affects 1.13-pre6. Please keep your issue updated.