If you play Duels now, you will be sent after the game in front of the NPC. I believe Auto Mode is kinda impossible for duels, as there aren't awaiting players all the time and it's already kicking you out of the game if you join and there's no one there to play against. It's working fine with the other games, because there are lobbies where you can stay and wait for others to join.
I was going to post the same thing but saw it was already suggested. Sometimes in games like skywars that have auto mode, you don't want to use it cause you want to watch the game or report a hacker. In duels since you only have one opponent, this is less likely. i understand that rematches could be an issue but just activate auto after a rematch is declined. Also, it's annoying to rejoin queue every single time, especially in normal duels that don't take a while.
Happy Halloween y'all! 
