Sigh. It seems nobody is going to look into the normals. I guess I'll have to do it myself.

My hypothesis is that Maya is exporting something with bad normals. The bug is in Maya, if there is a bug.

When you do that end-cap cut and paste, it isn't just that you're unwelding, you're also changing the normals.

The MayaCylinder that TheOwl posted has "vn" vertex normals in it. I don't have time to study them right now, but my guess is that one of them has a magnitude that isn't 1.0 or has a direction that is wrong.

The arrangement of polygon faces and welds doesn't seem to have anything to do with it. When I load and save from Blender (removing normals) the SAME topology is there. It renders fine.

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Renderosity forum reply notifications are wonky. If I read a follow-up in a thread, but I don't myself reply, then notifications no longer happen AT ALL on that thread. So if I seem to be ignoring a question, that's why. (Updated September 23, 2019)