I had this happen this week, too. There was nothing different in how the reservation was made versus the hundreds of other reservations where the updgrade default worked.
The PM Line told me they would add my to the UG list, but I didn't show on the list after the call. The DL app instructed that I needed to talk to an agent. When I got to the airport (LAS-ATL, 3:05 pm flight on Tue) my name was added to the UG list, but I wound up falling short for the battlefield UG. Not a big deal, I made it home and had a aisle exit row seat, but I do wonder if I would have cleared five days out had I been on the list from the start.