Unfortunately, I have had no luck. I set up a group inbox, with email-in and staged users, and it works as expected. There were no extra ‘Urgent!’ settings in the Tickets plugin that did any secret magic either. My email preference has stayed on Never, and refuses to flip.
Sorry I couldn’t be more help. Though if you can think of any other repro steps I could try I’m happy to give it another run-through.
If you have unused plugins you could try cleaning up your app.yml (and rebuilding) and see if that makes a difference?