I didn't see this posting earlier: from what I read, the (reporting) situation is slightly ambiguous but the good news is that your case is cut & dry - the bad news is but not in your favor.
Until your court supervision is up -- in a year, since this just happened unfortunately -- the SOS will report the DUI on your driving record with the court supervision noted in the remarks. This means that UPS now knows that you plead guilty to a DUI (even though you weren't legally convicted) and courts have ruled that they can legally hold you responsible for it. What becomes ambiguous is that a year from now it'll be purged from your driving record, but still available in alternate public records (as DUI are not expunged). Which records UPS purchases access to would be the unknown, but it's not relevant since they ran your record, it showed up (because it's still under supervision), they know about it and can hold you responsible for it. I wouldn't bother filing a grievance. If you need reassurance, talk to your business agent instead - I doubt you're the first person to run into this.
About ten years ago, on the night he turned 21, a friend of mine got an OWI in Iowa (DUI equivalent). At the time, if a first-time offender paid a fine & signed-up for classes, the offense was completely expunged from his record. So much that when he got his second DUI a year later in a neighboring state, he was once again treated as a first-time offender. I believe the laws in IA have since been strengthened.
