Yeah, another bug. All the dungeons have these bugs. First, the achievement for having all members of the dungeon be the same element can't be completed when there are only two members in the firepower team. Then, the achievement for having all members be guild members can't be completed in solo or with two members in the firepower team. What are you thinking, Bungie? Do you think this is fun?
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The bug for the subclass one is that it does count for solo and duos, rather than being exclusively for full fireteams like the triumph tells you. If they were to fix the bug, it would be to stop the triumph triggering at all for solo/duo lobbies. Clan triumphs are working as intended, it requires a full fireteam which is 3 players.