Something similar has happened to (friends of) my Scientist Sims ( I have two households with Scientists, one has a clone of himself in the same career).
They've crafted the Ghost Goo serum and tested them on other sims coming to visit or during social events. When the serum worked, the 2 test subjects turned into ghosts, but once they left the party/lot in ghost state, they do not return to their normal state. They are dead for all intents and purposes:
- I can visit their homes (both live with a living spouse, each in their own household)
- They can be asked "what it is like being dead"
- They can be befriended at their homes, but not called for a chat, invited on a date or party.
- I've tried making ambrosia at their house, but they did not choose to eat it - they took food from the fridge.
One of the test subjects is a townie who married into my original sim household before becoming a ghost, the other was an original sim I created.
Other tests where the Ghost Goo Serum worked, but the effects dissipated before the end of the visit/party left the Sims unaltered. It seems to me that if the sim leaves the active lot before the end of the serum's effect, the effect becomes permanent.
(I use Origin with DeLuxe & Get To Work, no mods or cheats.)