Monsters imprisoned by essence moving and attacking but invulnerable

Encountered this in Keth where a group of mobs was imprisoned but they still attacked and followed me when I got close but were invulnerable until I broke the prison. They were visibly glitching and teleporting back to their original position if I moved too far away. Video of what it looked like can be found here: https://www.youtube.com/watch?v=MjcaGSz2Lpo