When I use the left edge for intersection, the intersection with the surface fails (It works for the right side).
On the other hand if a have a independent curve on the right side the intersection is also wrong.
There is a collapsed edge at the point where the error occurs. BugIntersecting.gh (23.1 KB)
The surface here has a singularity where the 13 control points along an edge are collapsed to a single point. However they are not EXACTLY collapsed they only close together. So I would run the SetPt command in Rhino to get all those control points to agree exactly then try this again.