Immortality potion backfire permanent?

I gave the immortality potion to a few sims I don't play and some of them turned into ghosts. A couple who turned into ghosts turned back to regular sims and now have the immortality sign show up sometimes. Some others are still ghosts and don't seem like they're going to turn back. It's been weeks in game. Is this a bug? I gave the potion to a sims I do play and she turned into a ghost only for a limited time and turned back no problem, but with no immortality.