Leaving unconscious thrall and walking few meters away result in them sinking into the ground.
Sometimes walking far away from render distance can fix the issue but not always.
Yes, just as you described - when there’s a bearer III corpse or a T4 thrall on the floor, you can never move your sight away from them, or even bat an eye, for they will be gone in a microsecond.
But there was once I KOed a thrall and left her there for 5 minutes, when I came back from fighting a boss about 300 meters away, SHE’S STILL THERE! Weird!
Similar issue has been bug for a long time and well before 3.0 update. Previously only way to see them again was to walk away beyond render distance then walk back and they re-appeared. Not saying this isn’t new issue, but something similar was ongoing issue for long time.