On the occasions that the muon updater does successfully run (which it only does intermittently and you can't tell because there's no update progress), when it finishes the display isn't refreshed to show the results of the check for new updates.  You have to quit and restart the updater (you'll get a notification/tray icon if there are updates, but if there aren't absolutely nothing changes).

This, along with other issues already reported, makes the Muon updater pretty unusable.

Reproducible: Always