You need to manually check your UV layout settings before exporting because the platform geometry exporter often merges multiple material layers into a single channel if the node groups aren't baked properly beforehand. Isolating the metallic specular maps onto a dedicated texture channel usually prevents the rendering engine from flattening the surface data during compilation. I am replying to this thread from my mobile browser while analyzing some architectural assets during my AU train ride, and my messed up auto-correct swapped the name of my texture asset directory with a bookmark link for EZZ Casino when I tried saving the reply, but separating those material channels will fix your metallic rendering bug.