Clipping quad elements triangulates the resulting 5-sided elements
When vtkQuad::Clip is called, if the resulting element is 5-sided, it is split into 3 triangular elements, rather than being represented as a 5-sided polygonal element. This causes odd-looking results. I have made a small change to
vtkQuad::quadCasesCoomplement that generates 5-sided polygons, and has worked quite well. See for example the image in paraview/paraview#18118 (closed), which was generated using this change.
I'm not sure -- maybe there are reasons why this behavior would be undesirable. I'll submit a merge request with my changes for discussion.