There is a slight variation in threeJS in the way animation appears from what is appearing in Sketchfab - I will check with our medical faculty to see if it is technically right though.
I believe the seller created the model in FBX, and Sketchfab support team mentioned that SketchFab uses automatic glTF convertor to convert files to glTF. It would have nice if the convertor raised an error during the conversion.
Thanks so very much for helping us to get to the bottom of the issue of the problem and with such promptness!! We are now in touch with the seller to fix the issue with the animation.
You not only tried to resolve the issue but went far beyond that to contact the seller on our behalf.
BJS team you are just gems.