That exact ghost thing happened to us last night, as well. They were on the top ring, flashed out, the map markers blinked correctly, and then they glitched right where we were all running.

Simultaneous "Whoooaaaa whoa whoa whoa"s from everyone in the voice channel, lol.

I hope whatever's going on is sorted out; I've got people absolutely refusing to run the dungeon until SE announces better instance stability.