I am having the same problem, only mine is affecting my pet grave also. It's a legacy family, so I haven't moved the family off the lot, I just moved the gravestones so I could do some building. It just started happening and it's only happening with sim/pet that I wasn't on the lot to witness the death.
EDIT: I removed all of my CC (I wasn't using it anymore) and repaired the game from the origin menu. When I loaded back up, a grave I'd placed and had disappeared was back, but when I went to place another grave, it disappeared like before.
Has anyone else tried placing a grave, have it disappear, save and exit the game, then load it back up? I'd test it again, but I don't want to risk a legacy sim grave.
