Hi 3image, well the triangulation is valid and should render fine, there doesn't appear to be anything like triangles leaking outside the boundaries. But the triangulator is supposed to have a heuristic in it where it tries to prefer less skinny triangles but something is not working well with that mechanism for your particular case there. It's unfortunately a very difficult job to make the triangulator have the same qualities as human judgment when it goes about its job.

So I'd say it's a type of bug but unfortunately in an area that is quite difficult to mess around with without potentially introducing worse behavior in other cases. I may be able to tune it up in the future though.

For the meantime, try the attached version where I've divided the top and bottom faces in half, it should behave better for you with angle = 3.

- Michael