The corpse tooltip doesn't include -1 for burials happening. Or maybe burials aren't happening at all with necrophagy? Which doesn't seem like it should be the case from the code. Anyway, take a look at this save:
What the hell is happening to the graves? They never get built? But they keep being queued? I don't know if this is a bug in graves or a bug in undertakers or a bug in necrophagy or just an interface confusion, but I literally can't tell what's happening with corpses here. Are they building graves? Why do the graves disappear instantly? Are people being buried?