A lot of things went into the last patch but not this, so either it's something that they have to work on for a patch in its own right, or they became aware of it as an issue too late for it to get into this patch.
I have noticed it happening in my new game, and it''s interesting because I can see the process more clearly in this game.
I've been playing a bit with the Caliente family.
The mother of the twins died of old age and appeared as a greyed out ghost in the family tree to begin with. Now Nina has died of old age and she appears as a ghost - but her mother is broken now so shows up as alive and elder even though she's long since gone.
The game is randomly spawning tombs but not for all sims. The father of one of my sims is appearing as an undead elder sim because the game hasn't spawned a gravestone for him, but it has spawned one for Morgan Fyres O.o, who I have never played. I only know this because their house became a crime scene and my sim was there investigating.
Patch has fixed a lot of things but I guess we hope for this to be mended in the next. This game has no undead trying to move into houses (yet) or messing up any other part of my game, but it is clearly a process in the culling mechanism.
In summary...
Sim dies, is greyed out as ghost...
Game tries to cull ghost
Game fails
Fragmented sim remains as an undead with no aspirations or traits.
Game moves on to the next generation.
Bug continues.
Also, it doesn't relate at all so far to the ones I've had the warnings about losing their connection to the physical plain. None of the sims showing up as undead in my family tree are ones I've had a message about greying out.
In this game I only have about 40 active sims with a max of 150 set, but it is still culling and bugging the dead regardless.
Fingers crossed a fix is not too far away...and preferably one that restores family trees to what they should be without destroying all connections and keeper ghosts.