Animation does not import properly with an existing avatar if there is even a minor difference in the position/rotation of just one arm or leg or finger in the 0 frame between the imported animation and the base creature/character Avatar.
This should not happen since the rig is exactly the same.
Wasted a lot of time trying to troubleshoot until I noticed one starting pose was standard T pose and the other had the arms lowered. Also had to redo the animation.