I assume this is old news by now. But, my Hancock managed to stay alive for a total of like 1 hour or less after I dismissed him to The Castle (he didn't even walk there, I just left him there.)
Whenever I visit, the settler count decreases by one and Hancock is nowhere to be found. I can only assume his corpse is laying around somewhere unless it actually despawns and vanishes entirely after an amount of time.
I'm very bummed out about this, and I'm hoping when this gets patched and he's set as an Essential that he'll be resurrected. Worst case scenario I'm hoping the patch just teleports/resurrects him back at Goodneighbor or Sanctuary or something. Please Bethesda, I don't plan on replaying this game anytime soon, so having him alive in my current 5 day+ save would be swell, thanks.