Applying a metalness map that is pure white (255, 255, 255) causes the rendering error seen below. Both spheres are identical save for the metalness map; the right sphere uses a pure white map, whereas the left sphere has a (254, 254, 254) map.
Both spheres have only one UV seam, which runs around the equator and nowhere near the error.
The spheres have solid color diffuse channels (no maps), all-black roughness maps, but no normal or emissive maps.
The error is not dependent on lighting.