Split and Boolean Split fails—why?

In the attached file I have two polysurfaces (Red: Closed, Cyan open).

If I try to split or boolean split Red with Cyan it fails.

But, if I intersect, I get a closed curve.

Yet, I can split Red and Cyan with the the intersection curve and joint to create a closed polysurface.

Why this weird behavior?

Problem Split.3dm (3.1 MB)

it fails because of the coincident surfaces, that’s why it works with the intersection curves.
I guess it shouldn’t be that way but is well known Rhino doesn’t like coincident surfaces.