This is probably information already known to Ducati owners who race their bikes but I'll put the conclusion to this fun little adventure up here anyway: as it turns out the sprocket side axle nut has a tendency to work it's way loose in spite of the lock ring. That appears to have been the issue. Thankfully nothing catastrophic happened. The importance of having a healthy lock ring in place just became glaringly obvious. I might even start safety wiring the loop ends of the ring together.