so ages ago i uploaded this fbx -
which had mirrored uvs. Using the PBR shaders, this had flipped faces where the uvs were mirrored.
later i uploaded this from the sketchfab exporter -
same model! the same fbx was loaded into painter as was manually uploaded to sketchfab , but now the face normals are all correct.
bottom line, whatever you're doing in the sketchfab exporter, would be cool if that could be part of the manual upload as well. some sort of correction is happening and i can't figure it out with export settings alone.