That is pretty odd. I haven’t seen that personally but I would only suggest using this as a last resort if you really cannot use UV0. The black flecks look like 2x2 block artifacts which could be from using ddx and ddy.
It also uses the vertex normal to derive, if something is weird with your normals it could be causing it. But its impossible to say without seeing much more information about the mesh. Ie, UV layout, vertex normals, world orientation.