Probably because Blender isn't triangulating quads the same way as Marmoset. Triangulation is a bit of a pain to maintain between softwares, hence it is recommanded to triangulate your models before exporting. Check the wireframe. Try to force the triangulation by connecting vertices the other way. Don't know if there's an…
Thanks for the reply @Noors Usually I make sure not to check any triangulation options unless its completely finalised so both Blender and Marmoset versions are not triangulated I'm afraid, all in quads. It's just so I keep the simple quad version whenever I need to change anything. But here is both wireframe in Blender…