Rendering anomaly with CorridorGeometry and GroundPrimitive

Any ideas why the following sandcastle renders the red GroundPrimitive in this way? I like the ability to draw the corridor on the ground, but I get this behavior a lot with my primitives - instead of smooth, rounded ends, there are strange patterns and sharp edges. If I remove the first point it looks ok again - are the points too tightly spaced? Is this something you’ve seen before? I appreciate your help, as always.

This definitely looks like a bug, I submitted the following issue to Github so we remember to take a look at it: