I've just had my first sim with the Immortal trait die of old age.
For reference, here is the save file -
https://drive.google.com/open?id=1tKfmvNscoUlPEpXf-p4pgYoW1Av1Hg0t
In Del Sol Valley, there's a family called the Whites in the Pinnacles. Retta White, her daughter Wanda and *her* daughter Shanna were intended to be my coven of immortal witches, where I'd be coming back to play them periodically as they aren't my main sim for that save.
All three have taken potions of immortality and have the Immortal trait. I just got a notification that Retta has died of old age. Wanda is also reaching the point where she's likely to pop off soon, so you could probably replicate the issue when she inevitably dies too.
The really annoying thing is that I wasn't playing the household when she died, so I couldn't plead for her, and I can't find her tombstone so I can't use Necrocall & Dedeathify to get her back.
ETA: Actually, I should mention that ageing at elder stage is acting very strangely in my game in general. I have two sims with Long Lived but without Immortal who've lived entire sim years beyond the point where I'd reasonably expect them to die, and I've noticed three other sims with no life-extending traits who have lived at least two seasons past where I would expect an elder on normal lifespan to have died.
