I leave the graveyard, run into his ambush in the boneyard. I fight my way through them because I'm not ready for this fight, and leave through the south exit of the map. I prepare, and go back in, and they're all gone (except for the corpses of the ones I killed on my way out). Now, I'm doing an all-oddities run so I immediately save and dash through the next map to his regular place...and he's not there either. He disappears from the game. Since he's not dead, I figure this is a bug.