Did you use avatar importer, and custom? If not, go to Plugins + Avatar Importer + Custom and then select it’s file. Then it should import correctly and you can animate them perfectly.
Update: after more testing it seems the FBX importer is indeed fine, the issue is with either scaling the model isn’t working correctly, or playing the animation on a scaled model isn’t working correctly.
You should try the animation resizer plugin. ROBLOX animation, when scaled, was always incorrect for me, but that plugin should fix everything perfectly.
Pretty sure this is because the animations dont get scaled. So animations with movement will be wrong for differently scaled models, but rotations will still be fine.