This seems to be somewhat related to MC-76184, but not entirely the same. If you have a floating Armor Stand (or any other mob) with a Zombie (or any other mob) passenger, the Zombie's rotation acts extremely strangely.

Here is a video demonstrating the bug.

I should note that guardians and spiders function logically; they do correctly update their rotation even while floating/riding an Armor Stand (or other mob). When riding boats, zombies (mobs that aren't Guardians) also update their rotation semi-correctly if the boat is on the ground. If the boat is grounded, the zombie updates its rotation in jerked motions, even though the model appears to follow the villager smoothly. When the boat is airborne, the Zombie's rotation functions as it does in the video, and the model does not even appear to follow the villager.

The pitch appears to be updating correctly; only the yaw seems to be bugged.