It's a bug, especially around alters of lilith. Sometimes enemies just spawn in the terrain for some unknown reason. And because D4 is truly 3D, them being clipped into the floor prevents them from moving, or from being pathed to. Additionally you can't attack them if your line of sight is broken from them. Sometimes you can get them to respawn and fix themselves, but unless you absolutely need to kill them for some reason it's faster to just leave.