A follow-up. The problem looks worse than what I initially expected. Apparently is not even related to the use of root motion + allowing rotation.
Just a simple root motion will desync server-client when is initiated on top of a rotated actor.
Here you can see 3 cases:
a) AI is on top of a non-rotated actor and uses root motion + rotation.
b) AI is on top of a rotated actor and using root motion + rotation
c) AI is on top of a rotated actor and using root motion with no rotation allowed.
You can see a print text on his head representing the disparity between server and client location (becomes pink when is more than 100 units). The further from the pivot, and the more rotated the floor actor, the higher will be the difference. The delta looks related to the distance between AI and the pivot of the ground actor, aggravated by the ground rotation.
If the AI touched a non-rotated actor, or an SM (that is not an actor), it will sync back, as you can see when I debug close to the border of the square.