This gotta be a bug, right? After troubleshooting it, I just can't imagine what I can have missed that would mess it up.
So it's probably something I missed.
procedural.web.v02.issue.hip
EDIT:
Seems the dev's added a convert line SOP in H15 that splits a curve/edge into separate prim's - but I would still like to know why the carve SOP doesn't work in this case.