@escape Might not be that bug but simply the message sent so you can strengthen their connection to the physical world which stops the ghosts from being culled by the game. Even if your Sims don't know the newly dead but have had the NPC chat to them your sims will get the message and I believe it sends a few times as increasing warnings.
See points below from the linked older post 'Culling mechanics':
Housed dead sims will spawn an urn or a gravestone even on an unplayed lot, even if they are NPC's you've never played and even if you are not on the lot when they die.
There is a countdown on the graves 90 days(hidden game mechanic).
When this is countdown is at 14 days the gravestone will get a new interaction called 'Strengthen Connection to the Physical World'
This allows you to keep the ghost in the game for longer. In game terms, it resets the counter on the gravestone back to 90 days.
If you do not choose this the ghost will be culled/returned to the netherworld - the timer will expire.
To further protect a newly made ghost they can be moved into a house (their original household if you want) and then put into 'My Households'.
Culling Mechanics
