Region Intersect behaves weird with voronoi/polylines

I have managed to trigger this behavior with the RInt component, and with the C# call.

When using polylines out of the voronoi component, the RInt will trim these to an exterior polyline defining the boundary: The resulting polylines are closed and only contained within the boundary. This is the desired result.

However, when the same RInt component (or equivalent C# call) gets fed polylines generated inside a C# component, then it only returns the polylines that are fully exterior to the boundary curve. Why does it behave like a region difference?

Just to add to the confusion: In this odd case, the Trim component seems to be doing the right thing, but leaves the polylines incomplete (non closed). I could potentially fix them, however it would be nice to fix the source of this problem.