I'd hazard a guess that it's tied to mirrored UVs. Might need to try a fully unmirrored head and see what happens.
Edit: OK, it seems like this might be an artefact of using GLIntercept? If I turn on solo mode and just run around using the standard game 1st person camera it seems to be smoothing across meshes properly on all sides. But when using the GLI FreeCam you get the weird shading issue. I guess it's related to how it doesn't re-render culled offscreen elements?
Edit 2: Yes, my bad, it was GLIntercept. I didn't realise it previously, but by default it has a lighting adjustment function and that seems to have been screwing the shading up. By setting:
AdjustGLLighting = False;
in GLIntercept\Plugins\GLFreeCam\config.ini it is disabled and the shading in the game remains working properly. Apologies for besmirching your honour @bead-v.