When the fact type is displayed vertically (rotated right) I couldn't change it from a 'one to many' to a 'many to many' by doubleclicking on the constraint to edit its roles. I was able to get around it by changing to horizontal display, changing the constraint, and rotating it back. I was also still able to select roles and rightclick to 'add constraint.' It was only when doubleclicking the little purple dash to the right of the role boxes.
However, I was unable to duplicate this on a new model.
To help with this elusive lil' bug, I've attached the 'flypaper.orm' file. I took my model and removed everything unrelated, testing to see if the bug remained. Hopefully this helps track it down!