I've noticed that 3 out of the 4 dwellers in my endurance training room think there is a dead body in the room when there isn't I've tried moving all the dwellers out of the room and moving them back I've tried putting in different dwellers I've even tried deleting the room and putting a new one in its place (serious waste of caps)none of its worked and it's affecting their happiness

Have you restarted your phone? I had issues with frozen dwellers, where I can't move them out of a room. I restart and that fixes it.