I didn't see any problems reported on reddit/discords about this topic apart from some REALLY bad cases of bad RNG. I personally had streaks of bad luck like you mentioned at the beginning of the season so I think it's just the game being very funny with you. It may be a placebo effect but I think you should just close the game and retry again later or when the helmet is available again because it looks to me like there are some luckier days

[quote]So I've been running the concealed void lost sector for the past 2 days on my Titan. I ran it 17 times yesterday wen it was legendary, and I've run it currently 13 times on Master. For master it states that enhancement cores are uncommon, and an exotic helmet is common, but I still haven't gotten an exotic and get enhancement cores every couple of runs. On the Legend lost sector I figured just bad RNG, but now nothing even after 13 completions on Master in addition to the 17 on legend seems like this may be a bug. Especially since I am receiving the "uncommon" reward so much more often than the "common" reward.[/quote] It’s a drop chance, you’ll have to keep running it till it drops, that’s what I did to get my exotics

Might be all those Wrathborn Hunts you are farming, messing with your rewards.