I’m wearing the Nightfall helmet (as you can see on my avatar), yet, in my collection, it’s not there. Is it a different version of Nightfall that’s in the collection screen? If so, why is the one I have not on there?
Probably a bug or a glitch of some sort. But its pretty weird. Doesn’t affect anything so its ok 
The promo REQs for armor only show up in your menu where you can customize and preview the armor. Stuff you pull from packs shows up in your req collection. It’s confusing but they are the same helmet.