In case this helps spark a thought for anybody:
I'm wondering if this has to do with some kind of flipped normal or tangeant direction.
So I checked my face normals and vertex normals. They all seem OK and are all pointing in the same direction.
When I display tangeants in maya however, tangeants seem to be rather unruly. I'm not sure if this matters. See this image:
You can see the red tangeant on two of the vertices of the faces that have a shading problem, are pointing in the opposite direction as all the others. Could this be a clue?
If I view them as vertex face tangeants, some of them seem to be pointing all over the place...
I've read that tangeants are dependent on UV space. As you can see from the normal map screen grab, the UVs are continguous, no funny business there.
Is this perhaps a vertex ordering issue?? Something in the OBJ export from Zbrush to Maya?
To compound the problem, I'm really struggling to troubleshoot this because of various things that feel like GPU bugs:
1. whenever I select a vertex, the shading in the viewport changes a little bit, as if the normals were getting edited (even if I don't move any vertex)
2. if I resize the object by typing a new scale value into the channel box, the viewport shading completely breaks
3. In either of this case I have to close the scene and re-open it and it displays properly again.
By the way, I had read that you can't do technically correct bake from xnormal to maya without using Handplane, so I got handplane and went through all the steps, but this made no difference. It feels like the error is something more fundamental and inside maya.
Please help -- I feel like I've run up against the limits of my knowledge here on what is ACTUALLY going on with a face's normals!