I played a casual game with Nozdormu in my Rogue deck. My opponent conceded before the mulligan. My quest for play Rogue games progressed. My Nozdormu quest did not progress.
Some quests require you play a certain number of turns before the quest’s counter can increase.
This requirement was recently relaxed for “play as class” quests, but may not have been for other quests.
bet that is the issue.
the Nozdormu quest is worded “Play 3 games with Nozdormu the Eternal in your starting deck.”
Its just a wild guess, but I bet the game doesnt check your “starting deck” until after the mulligan.