Since the latest update my dwellers have stopped floating through floors (yay! That was eerie.) But now some ghost during attacks and become invisible. Usually during DeathClaw attacks. Their sad power bars are visible getting lower and lower but I can't heal them, see them, or move them around. They remain invisible and unmanageable while dead. My workaround is to sort (197) dwellers by happiness, go the bottom, click on the dead dweller, then I get the dialog box to remove or revive.