Having both custom vertex normals and the "is the face sharp?"
attribute on a mesh leads to some non-intuitive behaviours. Instead,
make the behaviour more aligned with OBJ format description wording,
which says "When vertex normals are present, they supersede
smoothing groups" (https://paulbourke.net/dataformats/obj/).
Pull Request: https://projects.blender.org/blender/blender/pulls/124366
6f0620161d