Screenshot below, it only seems to happen on this Mountain Aerie tileset, usually near where a wooden bridge connects with the rock floor. The flame wraiths make all the gestures like they’re throwing fireballs at enemies, but nothing will leave their hands and the enemies just walk right up to them and mow them down.
Update: Ran into this again, and it doesn’t seem to be the floor or the bridge that’s the problem, but the giant bonfire. Whenever Flame Wraiths are placed near the bonfire they bug out and are unable to attack. I tested with several different bonfires in the same Mountain Aerie echo, and near all of them the flame wraiths displayed the same behavior.