all characters with blendshapes have distorted teeth and inner mouth parts. So it seems all characters coming from the same base, all have the same problem. Open the mouth, it looks very ugly and is not usable. No matter if the models are opened in 3dsMax or Unity. Any solution? Or could the morphtargets/blendshapes be reworked, so all (visible) distortion of inner mouth is fixed?
so how does 3dsMax or Unity have anything to do with Mixamo?
the teeth are different in Fuse models which is what your screengrab looks like so the distortion is prob happening at the import stage