The horizontal rotation (y_rotation) of zombie and skeleton that spawn by command will not update until the mob is aggro at something but the mob itself can still move their head like normal.

This seems to affect everything that involved rotation

/data get entity <Skeleton> Rotation
/particle happy_villager ^ ^ ^2
....@s[y_rotation=10..90]....
etc.

This is only happening with horizontal rotation, the vertical rotation still acts like normal.