maybe this is interesting to debug for you:
getting these 2 surfaces out of grasshopper and after joining the bottom surface won’t shade.
the only thing that helps is removing the inner trim.
the trim curve is an intersection between the two surfaces.
I tried to rebuild and refit it before making the surface split, but it
won’t change the shading problem. So I think it bight be some rare glitch here.
meshError.3dm (966.9 KB)