I dropped onto the Moon and joined a Tier V Altar already in progress. When the final Altar boss was finished, a triumph popped up on my screen, but could not find it to redeem. I thought, "it's Bungie /shrug." Braytech also thinks completed one, though; the description is completely blank except for the Solstice icon and 6/6 completion.
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Probably something for summer solstice event that triggered early would be my guess. In the past, upcoming event triumphs have procked early.... only a guess though.