Retargeting fail

My issue:

  • Left side: a regular mixamo t-pose mesh
  • Right side: my .fbx with 2 animations without mesh, imported as an animation library

As you can see, the animation is off, and also the mixamo model right-arm seems to still be in t-pose.

I import both with a bonemap + skeletonprofilehumanoid and the bones are correct in both. I then retarget the animation, by introducing a new AnimationPlayer on the mixamo node and picking the animations from my source .fbx. The animations play fine, its just that mixamo likes to be in t-pose and doesnt follow the source animation properly. It does follow the animations somewhat, just wrong.

This is how my source .fbx looks like in the importer, note how the right-arm is not in t-pose.

My question: what is wrong here? I suspect its something with the rest pose not being detected properly (I dont have a rest pose in my source .fbx) ?