This happened to me while playing Creeplight Ruins (spent 100 gold on preparations, sigh...), when I killed the sacrifice goat. Before this I had an bug where a Cultist jumped 2 squares away from me, then I killed him, and then the zombie appeared between where I was and where the cultist was, I couldn't attack him because he was pretty much just a poster on the floor, and there was an invisible cultist still alive on that second square whom I couldn't kill. I suppose the game tried to spawn a wraith right there, or something.
Are the cultists supposed to sometimes jump 2 squares away? Or any other cowardly enemies, for that matter?
If needed, I have another screenshot here with that full log visible.