I’m having this issue as well. I noticed that the Sims whose gravestones I cannot place have also disappeared from my family tree. FWIW, I’m doing the legacy challenge. The tombstones that cannot be placed are children from the second generation of the challenge, and I’m currently playing a Sim character from the 5th generation. Basically, her great-grandfather had a brother and a sister, and they have been the source of the issue. Is there a bug with family trees that erases a Sim from history after X generations that could also affect the tombstone?
Edit: I also do not have any mods or CC
I also have the same problem. I have 10 gravestones in total, but can no longer place three of them.
Have the same issue with a gravestone not able to pick up. Sim died of heat stroke and was an elder.
I am also having this issue. I didn't change lots just a new house on the same lot and now I can't replace my tombstones
