On the map Volcanic City of the Monolith of Faith level 62 timeline I had the “Cleanse the Acid Forge” quest. Enemies would spawn behind a fence, where they could not move. At first I didn’t even see them, thinking the forge was somehow broken, because there would not spawn a new wave.
https://imgur.com/fkiWJ4C
Here you can see a hit point bar in the lower left corner of the image - this enemy cannot move out of there (but in this case can attack me with a ranged poison attack).