Blender to Unity Mesh Deformation Animations and model looks great in blender. I import into unity and it this deformation happens and I can't seem to get rid of it.The animations all play correctly in unity but the deformation on the hand is always present.
Things I don't know about 


This model/rig uses plenty of IKs in blender so I am not sure if that translates into unity although i haven't had problems from it in the past
This model also uses the child-of constraint in blender and in the animations the influence value is changed.


Things I have tried. 


Applying and reapplying LocRotScale
Removing re-applying armature
Messing around with unity model settings (i.e. rig from generic to legacy, mesh compression etc.)
Double checked weight painting and all vertices are correctly assigned.


the model can be downloaded here
The default importer is doing no such thing for me when I put it into Unity with the default (click + drag) importer.

I suspect that the issue is purely on Unity's side.  The version of Unity I am using is 
2018.3.2f1.  You might want to try updating your Unity and Blender versions.