Ideas
My horse died of old age when I was on a different lot. I got the notification as part of the Life and Death expansion pack. When I went to the horse's household (part of a residential rental property), there was no tombstone and the horse was no longer part of the family. My main sim showed the horse as alive in the relationship panel, and I was able to "invite to lot". My horse showed up alive, with the "elder" lifespan. There was no option to add to family, and she did not show up in played households, nor other households. My game did crash once when attempting to do a competition with the horse, but may have been unrelated because I was successful joining one as well. I play on Ps5, so I use no mods. I do not have testing_cheats enabled in this save.