ive completed maybe around 8 or 10 1280 solo lost sectors today
and i have not gotten a single exotic helmet
either this is 100% bugged and not dropping as commonly as they should
or rng has absolutely said "screw this guy" and decided to not drop me anything after 8 or so runs
all of these runs were under the 15 minute darkness timer
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Keep going, it’s rng. My clan mate ran it 20 times before getting his exotic. But my husband went in & got it his first run. Hope you get it