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
I have done a bit of testing, on my hunter when I run a 1280LS in the past I get an exotic almost every run 8/10 odds or so. On my titan I have done 20+ runs and still have not received an exotic, when I run 1250 lost sectors my drop rate is somewhere around 1/3 on all characters. so definitely something is bugged.