Ah, see, that's the artifact of what happens when you do this.
First, you resurrected the sim. Of course, the urn on the original lot cannot be touched until you actually visit that lot. So the urn remains intact there. However, before ever going back to the lot, you killed your sim again.
Now he's dead, and there's an urn on the second lot.
Back to the first lot: The urn is still there, the game sees your sim is dead, and does nothing about the urn, because it's associated with a dead sim. You delete that urn. Deletion of an urnstone causes the "Unlink Character Data", irrevocably shredding that character's file down to a GUI representation (picture).
Return to the graveyard: Urn there now associated with unlinked, invalid sim. Initiating self-destruct.
Translation: Don't do this. It causes duplicated death memories and wigs out your game somewhat. If the "Graveyard Maker" doesn't actually appear in NL, I'll probably make an Urn-mover to let you BRING OUT YOUR DEAD.