This bug relates to MC-96365 and MC-244804.

Steps to reproduce (a video is attached):

1. Make a superflat world with blue ice as the top layer. This is not really necessary, but it makes the bug more evident.
2. Place a boat.
3. Enter the boat.
4. Press A or D until you reach the maximal rotational velocity.
5. Exit the boat.
6. Note that it stops spinning due to MC-96365.
7. Place a mob into the boat. I used a husk.
8. The mob's head begins to spin forever. (I only tested this up to several minecraft days, but the rotation did not stop or slow down.)

This bug can also be reproduced by putting the mob into a boat first, then entering the boat, then accelerating, and then leaving the boat.