(Android, Galaxy S6)
So after getting in a fight with some mole rats while my strongest dwellers were out in the wasteland I wound up with a couple dead dwellers. I'd only just been starting out so I didn't have a lot of caps, so over the course of a day or two I struggled until I had the caps to revive them(I didn't have a lot of dwellers, either, and getting more would take a lot of time so revival was a better option at the time, imo, than removing them.
After reviving them, however, I had low water and food and power and a lot of my dwellers were sad. I had like 20% overall happiness and things were a mess. But I pushed through. A few hours later the radiation was mostly clear, everyone was alive, most people had full health, food, water and power were good. . .but a bunch of healthy dwellers were still upset. I zoomed in on the room they were in(the power generator) and they complained about a corpse in the room(except one, the last one to have been revived, I think--he was happy as could be.) I lost like six dwellers because I wasn't fast enough to move dying ones out, so I would have perfectly believed that there were still some left over, but I scoured the whole room and everyone was alive. Everyone in the vault was alive, everyone on the dweller list was alive. There were no corpses to complain about. I moved them out the room and back and no dice, I moved the one guy not complaining out, nothing, I swapped dwellers out and they had the same complaint, I believe. So I had a bunch of happy and healthy dwellers stuck at 10% happiness because they were hallucinating a corpse. Cool. And, of course, moving them out, cheering them up, and moving them back in just had their happiness drop again.
I don't know if the problem still persists because I got raided and attacked by molerats about an hour ago and lost a bunch of dwellers in the same room and have to work on getting them revived again, but hopefully this can be looked into. I dunno if there's supposed to be some sort of residual death effect that's causing their complaints but I'd assume not since I've had dwellers die and sit until I revived them before and this didn't happen. It's a fully upgraded three-room power generating room so removing it and then replacing it's kind of not an option I'd like to resort to considering how little you get back(and considering how big my other rooms are and how much power they take up.)