I am having an issue on most of my flat surfaces going from Substance Painter to Marmoset 3. It looks like this is centered around the triangulated edges for some reason, making the mesh look "squishy" in Marmoset. I have tried a number of things, from forcing my Max export to Marmoset to not triangulate in Marmoset, to…
It looks like your lights in marmoset are way "sharper" than the light in substance painter. Try to increase their width if you are using additional lights beside the environment. That will increase the reflection area when the lights are visible on the surfaces.