I had something very bizarre happen with a gravestone but I doubt it's related. Nevertheless I'll add it here because you just never know.
The Ambrosia family were moving to a new house and of course they wanted to take their Leo-the-Romance-sim-sunburnt-vampire-ghost with them, so they packed him up with the rest of their goods.
The instant their lot loaded, out popped a gravestone right in the middle of the sidewalk! I thought, 'Well, at least I know where the cemetery's going,' and picked it up and moved it there. The description looked great: "Here lies Leo Corsillo," so I neatly placed the stone and placed the two bouquets of roses that stand on each side (he really was loved, you know).
When the family moved, I let Alistair be the keeper of all things marvelous and wonderous. In his possession he had all of the aspiration rewards (thinking caps, noodle soothers, Renu-u-Sensor Orb, Elixirs of Life, Leo's flowers, the gravestone, career rewards, etc.). Shortly I decided to go ahead and unpack all those goodies and something struck me very odd. Right there in Alistair's inventory was a gravestone! I took a double-take to see if indeed I had placed Leo's stone and sure enough I had. I took the one out of inventory and checked the Buy Mode description, which said simply "Rest in Peace". I thought, "Great! I don't know for sure which of these stones holds the ghost! I'm assuming it's the one with his name, but as wonky as this game is you just never know!"
So, I placed the newly recovered stone up by Leo's stone.
Well, today when I restarted my game and went to the Ambrosia household, the extra gravestone has gone missing. I've no idea where it is, but apparently it wasn't needed because Leo has wasted absolutely no time going haunting and trying to scare everyone out of their wits (like it's their fault he got a little sunburnt?)
Like I said, this may not even be related, but since you were on the topic of disappearing gravestones I thought I'd add my 2-cents worth.