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!
A bug, caught.
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user_id=1452263
Originator: NO
flypaper.orm has nothing in it, a real file would be useful.
Are you sure a reading shape (or some other shape) is not obscuring part of the constraint?