I think I had this bug twice now.
Arena, best of 3. Monster won first game. 2nd game was very long. Hunters managed to win, so 1-1. But the “Hunters win” screen never disappeared and then the error message “Connection to session lost” (or sth like this) was displayed.
I think it has sth to do with how long the 2nd battle took. But it also could just have been a random disconnect… Anyway, the connection loss was very frustrating because it would have been an epic final battle.