There is a glitch on the mastery class for the triumphs, there is actually more than 15 triumphs(17 now) in the weapons section. I got 40 mastery triumphs completed today and didn’t award me the emblem as it was supposed to. It stills says 38/40 even though I have 40 out of 42 available, the only 2 remaining are the mayhem triumphs as mayhem hasn’t been a featured playlist on Season 4
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It’s not a glitch. If you read it correctly you’d know the persistent blaze emblem is only obtained after completing the mastery category. which is obviously impossible at this point in time. As you already pointed out mayhem isn’t currently a playable game mode hence you can’t complete that triumph and therefore can’t complete the mastery category until mayhem is back. I do think it is stupid on bungies part to lock it behind something that isn’t even in the game right now though and I myself also want that emblem.