Sul Sul, @EA_Arrenai.
I came here with a similar issue, only it was for a sim that wasn't in my household. I gave him a potion of immortality that turned him into a ghost, and many days later when he asked to come visit, he was still a ghost. I tried to Dedeathify him, but that failed. Came here and read this thread, so I transferred the ghost sim into my household, and fed him some ambrosia. Cool animation, but it dropped him down to the floor still as a ghost. I made him a magic user and overloaded his charge many, many times trying to suicide to see if the Grim Reaper couldn't fix him. Alas, he never died because I guess you can't kill a ghost. I then noticed that although he no longer has the moodlet talking about the potion of immortality failure, he does have a permanent trait referencing the dangers of pursuing immortality, whose tool tip mentions it will "pass in a while." It never did despite spending 2 or 3 days in the Magic Realm trying to suicide.
Hope that helps.