Wow, Arena is broken beyond belief now.
Went to a Quick Constructed queue, played 1-2. Then on the fourth game I won, but the game froze when I did lethal, literally when the opponent was at 1 life and I played Sunscorched Desert. The game started roping me so I restarted in the hopes it was a connection issue. It wasn't. Instead the game restarted, automatically put me in a match loading screen, put me against that same opponent, and drew us the exact same cards, starting hand and topdecks all.
The "re-match" was unwinnable because of perfect information, so eventually I scooped and was put back in the main screen. The crashed game session had registered as a loss (of course), and apparently this phantom game was not part of the QC at all.