This seems like odd behavior, so I thought I would mention it.
If I do a FilletEdge and it creates caca then UNDO, the caca surfaces remain in the file.
Undo removes the fillet surfaces here in 5.1 (5B161). Can you send in the file before and after to tech@mcneel.com ? Or post it here?
Let me see if I can get one. I should have said that undo does not consistently delete edges, not that it always does.