Weird Lighting Across The Middle
I am working on a project with this model, that I am told comes from somewhere on sketchfab. As you can see in the second photo, this model has some weird lighting effects across the whole middle of the body. These lighting effects do not appear when smooth shaded and only do on flat shaded. These creates some problemas for my purposes.
I have already considered several possibilities and fixes:
Vertex issues: Vertex may not be welded, or they may be some extra vertexes there. So, I tried merging by distance and checking each vertex manually to see if they are nicely connected: They are.
Normals: I am not very knowledgeable about normals, so in the the first picture you can see how they look across the problem area. I tried recalculating and manually rotating but both seem to have no to little effect.
Mirror modifier: I tried deleting the second half of the model and recreating it with a mirror modifier. That didn't fix the issue either, and actually made it kind of worse.
The normals across the middle are slightly tilted to the left but manually rotating them to be straight do not fix the issue. When I was recreating the half with a mirror modifier, I saw the open model, and it had a wall across the middle section, but deleting it did not fix the issue (see the picture below)
What would you guys propose to fix this problem?