The transparent segments are a rendering glitch associated with convexity. Although convexity is best known as an issue for linear_extrude, render, and STL import, it affects any case where CGAL gets involved in the rendering and can return a concave object.  In this case, minkowski() is the culprit.  Although the convexity parameter is not documented for minkowski(), it *is* implemented.  Changing the minkowski() invocation to set convexity - I used 5, but that's probably overkill - makes the glitch go away.