I've done all the Battlegrounds missions as they came out with the season's storyline and then repeated them individually through the Battlegrounds strike playlist, however, this triumph still says that I've never completed the Behemoth one, even though that is incorrect. Since this triumph is a required one for the seasonal seal, then that means that my chances of getting the Chosen seal have been greatly reduced by no fault of my own because now I have to get lucky in the playlist to get the Behemoth Battleground to complete and I also have to get extra lucky after completing it for the game to actually give me the triumph. Having a bugged triumph that won't complete isn't anything new, however, when the game wouldn't let me complete the triumph to gather all the Moon campaign lore books it didn't bother too much because that triumph wasn't tied to anything. In the case of the "All Battlegrounds Missions" triumph, I am being locked out of receiving a seal because the game won't acknowledge that I've done the Behemoth one.