Material blending of some sort will likely be added down the line. Yep, Maya is in there now and works perfectly, Xnormal/Blender (Mikktspace) is about 95%, Max is in but totally broken still. The "default" tangent space seems to do a much better job as well. Its a per-mesh setting in the object properties. Certainly…
It's because on metals, diffuse isn't needed in 95% of the cases, so for metals the reflectance values go in the albedo map instead of just black (and then in the shadere gets replaced by black where the metalness map is white). For non-metals, the range on the reflectance value is so small that it just gets replaced by a…