It remember me on an old problem. So far I understand there are different ways to handle meshes and the Rhino system is quite limited. Some years before Michael from MoI3D explained it at post no 3 of this thread:
I don’t know so much about this topic but maybe the old thread helps someone to find a way to change the handling of mesh data at Rhino. The old thread is about shading normals at welded edges, but I think it’s the same problem for UV coordinates at edges. Could the mesh system not be changed for Rhino 6?