Just wanted to say that I reported this years ago because it kept happening with my vampires. Only after one of the recent updates the game's forgotten who the tombstones belong to.
Basically when a sim is dying, while the death animation is playing exit to the main menu and reload your last save for that family. Then a tombstone for the dying, but now still alive sim will appear in your active family's inventory.
And when you do that a few times you can make stuff like this (these 5 vampire tombstones are all for the same sim):
I submitted my save back then, but was told they couldn't reproduce it, which to me is weird because it happens for me every single time I do it.
I just recently had this issue myself and can say using Debug Reset on a sim before grim can do his thing then choosing Exit to main menu causes the exact same issue (Urn and all past items being in household inventory and you get System Error if you try to place other than that game is still playable). My fix was rolling back to a previous save. Deleting the cache and all normal stuff didn't work.
I play on PC... This is my save if it helps it contains the Recovered version without the issue as well as the one with the issue https://drive.google.com/file/d/1TTVHppPwXgp7J3EM1pfqr5WlFCyHmw8s/view?usp=sharing
