So the issue here is that the UV map in the FBX has some real garbage in it, there are area where the triangles overlap themselves, and this makes it impossible to bake unique pixels for those areas. It appears that the top and bottom where unwrapped using a planar UV projection, and this unwrap included the “side” parts.
In your last photo it looks like you have not just generated new polygroups, but also generated a new UV unwrap, which, yes, will mean that the texture bakes can work properly. However this new UV unwrap isn’t in the source FBX, so you would have to export this asset as a new FBX file to use it in Mixer, so that the UV map you are importing into mixer is consistent with the baked texture maps.