I had just finished an objective and recieved a lunchbox that contained a dweller wearing a noble robe with good stats, and also had two other dwellers waiting outside my vault. To get all of them in quickly I drug them to one of my rooms with only 3 people, and as they came running inside, I went to inspect the dweller I recieved from the lunchbox, to see what the robe did, I got pulled away from him by accidentally hitting my destroyed mr. Handy, and when I went to go back, he had disappeared. He is nowhere to be found in my vault or dweller list, though the "noble robe" or whatever he was wearing showed up in my collection. It's like he glitched and disappeared.

you probably removed him by accident