I completed the most recent Nightfall, had a 100k score, was using Bad Juju, Beloved, and Fixed Odds for weapon, and had Opulent Stalker mask as well, but still wasn't awarded the Triumph.
Seen a few other posts about it being bugged as well, but not seeing a resolution other than just trying it over and over again. Hoping someone has a fix.
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Not bugged. You need all weapons/armor to be of the correct variety. You state you only wore the mask (but not the gloves, chest, boots, or class item).