Thanks for looking at that. First I made the mesh using 3D face in rhino WIP. Non flat quad are triangulated.
A simple example of the problem
1 Make a flat quad mesh in WIP move a vertice to make non quad
2 use these vertice to make the same mesh
So you have 2 meshes with one face
Apply GH script,one will have an extra edge
It is surely normal but when I draw a quad it is because I want the quad edges not the edges of the triangulated quad/ngon.