Ideas
Ultimate Gauntlet is almost over and the problem persists. I tried leaving the game mid match but just got a warning about not finishing matches. Still made no difference to my progress in UG. Realistically, I cannot finish the objectives now so have missed out on any potential rewards through no fault of my own. It's farcical. All other game modes are working fine as is my connectivity!
Did this happen to you again in a new gauntlet session started after closing the game during the match that keeps getting disconnected at the end? If you only tried abandoning the match, that might not be enough to get things unstuck.
Sorry about any rewards you might have missed out on, I understand the disappointment. I hope you'll be able to start a new gauntlet session following the suggestions I pinned above.