Because in Rhino 6 meshes are more strict. Your triangles are quads with a collapsed edge. You can fix it with the small c# script I provided here: Mesh Surface (Mesh UV component) makes Invalid Mesh with Spheres (Rhino 6 + GH 1)
Alternatively in your case, if the goal is single mesh faces you can just use the “Simple Mesh” component.
The “Mesh Brep” component won’t give you single faces always.