So I've had this problem for a while but I haven't seen anything posted about it: I currently can only track 4 triumphs despite the cap being 6. I started having this problem after Solstice of Heroes 2023 because I was tracking 2 Solstice event challenges. So when they disappeared at the end of the event, I believe the game still registers me tracking those challenges despite how they are not being displayed.
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Bungie are aware. https://www.bungie.net/en/Forums/Post/263186794?sort=0&page=0