FBX uses https://docs.blender.org/manual/en/4.0/addons/import_export/node_shaders_info.html (PrincipledBSDFWrapper
in scripts/modules/bpy_extras/node_shader_utils.py
in this repository), which…
Umm, I'm not actually a Blender developer, just a contributor, so I don't think I have much say over new addons.
Yes, I can no longer reproduce this either, so it appears to be fixed in 4.1 now.
I think this may be a case of Blender rigging not handling custom normals as well as other software. The FBX importer simply imports whatever normals are in the .fbx as custom split normals…
@ideasman42 is this one you can review?
Thanks. I realised the functional change to new meshes after submitting the PR, but wasn't sure if it mattered in this case due to ".edge_verts" being an attribute that can't be removed.