I have played Gambit before and after Witch Queen. I wasn't able to get the Seething Heart before, so it was good to see Malfeasance updated to dropping on any win. I have won games/defeated the primeval, including getting a for sure final blow and have yet to have the quest drop. I'm not sure if this has anything to do with it, but I have yet to receive the Xbox achievement for winning a game of Gambit, as well.