This just happened to me as well. Opponent conceded game 2 in Traditional Standard. Instead of getting the Sideboard UI, I was left on the game board. The settings gear was gone. I alt+f4ed and relaunched, and then after logging in, I would get stuck on a screen that looked like it was trying to find a match. I alt+f4ed several more times and eventually I just got taken back to the main menu. It appears I took a loss for the whole match since I lost pips on my rank.
This just happened to me as well. Opponent conceded game 2 in Traditional Standard. Instead of getting the Sideboard UI, I was left on the game board. The settings gear was gone. I alt+f4ed and relaunched, and then after logging in, I would get stuck on a screen that looked like it was trying to find a match. I alt+f4ed several more times and eventually I just got taken back to the main menu. It appears I took a loss for the whole match since I lost pips on my rank.