Yep, this is a problem with the metalness workflow, and storing both diffuse and specular content in the same map. More here: http://www.marmoset.co/toolbag/learn/pbr-conversion
It's less visible on a surface that's already dark like in your example but if you do it on a rather bright surface, you'll see a darker outline on a smooth transition from non metal to metal: The diffuse is getting darker on those edges and the reflectivity is getting brighter, so you end up with the same issue as when…