Even though everything deforms just fine in Blender, as soon as I go to export it into .fbx and import it anywhere else the model is just obliterated - mesh is all out of wack and the UVs are incorrect.
Deleting the armature modifier fixes the geometry (though I need it, obviously). But, the UVs are still wrong and the model seems see-through - and it's not a normal issue, all the normals are correct in the 'face orientation' rendering mode.
I did read a post on here saying that merging a rigged mesh could cause issues with the rigging and/or shape keys, but I joined all of my separate parts. Not sure if that could cause an issue or not. Plus, the armature and shape keys behave correctly in Blender.
I also tried the Better FBX addon, and that doesn't fix the issue either.
Does anyone have any idea where to even begin debugging this?