A short to one of the magnets/poles can be detected with an Ohmmeter.What I have found sometimes the magnet gets rusty (?), and that swells and breaks the thin insulation or the wire. And makes a short to magnets.
This all makes sense. Would most probably cause shorted turns and would make PU sound dull depending on number of turns involved.Or the insulation deteriorates for some reason: age, moisture, sweat, vibration...
That should make the PU read open ("OL").Or the wire was winded so high tension that it is just at the edge of snapping. And then snaps after sometime, years, decades?
You forgot to mention one common reason: Someone tried to push in a magnet. Especially risky with lacquer potted PUs, where the wire tends to stick to the magnets.