This is with edge split on, removed duplicates, faces are quads, no backfaces, etc.
I’m totally stumped, I’ve even tried marking edges as sharp and no solution. 
No blend file supplied in your post !!!
Please ALWAYS supply a blend file for all support questions so anyone who may be willing to help you does not have to just use guesswork !!! It’s extremely bad mannered
Check face normals,
Check for any multiple materials
Hi, can I see the object in edit mode? have you try to select the face with the issue and invert the normals? only with that face selected