In Queen Silandria's room there is an elemental doomseeker spawn... if you aggro this spawn and run back down around the bend outside of where avatar of smokes room is instead of going through correct pathing this mob will instead go through the wall and glitch under the world like mobs used to before pathing was added.
Have had this happen a couple times, both times pulling w/ selos on as well if needed for testing purposes. Either way the mob needs its pathing fixed.