Couple things that perplexed me...
1) UA agent called over to confirm the space before booking, but I guess he may not have told them it was an -UP fare.
2) When I called the DL plat line, she looked at the ticket and said "Oh, I see. This is an -UP fare, we have them too. They can't be booked into F, but there is A available so let me call over to ticketing to get this done." She then came back and said it was all fixed and there should be no further issues.