I’ve farmed several Magi rings, and I’m positive I’ve gotten a couple Nightbringer’s and thrown in my Item Assistant.
I was just thinking of a creative build to use it for, probably not a super great build… but I went to look for the Nightbringer and none to be found, lol. I even logged on every single level 100 hero I have and checked everything. Nothing. Seems weird.
Oh well… guess I’ll do some more Magi runs… lol
you sure it was Nightbringer and not the blue one? (diff name)
no idea how item assistant works or stores/retrieves items - but if it’s anything like GDstash, if you were on HC/SC it might be an issue of not being in the same “mode”?
likewise, if an item is stored, visiting chars wouldn’t bring them up - tho then again they should appear when you searched in the IA storage at least i guess 
only times i’ve seen items disappear is
on crash, effectively before the game got to save it
when accidentally overwrote/deleted stash tab
when stash file got messed up/progress reverted (steam cloud)
otherwise, once an item is “saved”, it’s saved, and it wont just vanish on its own “while no other things were effected”,
likewise when (GDstash) item is stored it’s stored - and if pulled out of storage into stash and saved it’s in stash (but obv no longer in storage)
Hmm, well, I do have the blue one as well. I mean I suppose I could be mistaken, but I’m pretty sure I specifically remember it dropping thinking wow this is kind of a neat item. But knowing my memory, maybe I just saw it glancing at a build or something.