Im pretty sure this has something to do will NaN values existing in the gbuffer somewhere, perhaps the normals?
A NaN normal would appear black typically, as when the engine tries to calculate direct lighting it would fail. And when a NaN pixel gets sampled during postprocessing effects, things can get weird.
I’m guessing something messed up with either during the mesh import settings, or there’s some defect in the mesh normal vectors.
If it happens regardless of material, it must be the mesh in some way. Look at the buffer visualization viewmodes, especially the world normal, and see if anything looks off.