Interestingly, I had somewhat similar happen to me the other day. I applied my global upgrade certificate, the upgrade cleared but it always said "seat assigned at gate." Five minutes before boarding the red jacket gate agent said I was confirmed and my new boarding pass would print at boarding. In those five minutes, I was somehow bumped to economy-class. Even my meal was business-class (special meal). The Diamond Desk supervisor says someone at the airport bumped me down. Delta is still investigating but in the interim I have the certificate back and 22,000 miles in my account.