Older Sims disappear from genealogy

I have their graves. Tecnically they shouldn't disappear from the genealogy, despite they have children or not.

maybe the "tie to world" (not sure the english name) interaction was too long ago and the game deleted it?

Did the sim disappear from the game or just the family tree?

Just from the family tree, because I have their grave. I've never activated neighbourhood stories with my families because I was afraid to lose their urns and can't make funerals. 

This has happened to me too. I am on Gen 8 of my legacy challenge, and most of the older generations are only showing the direct ancestors now, all siblings are missing. For example, in Gen 2, there were 3 children, but only 1 is showing in the tree. It is the same for following generations. 

Zaerinelle, is that perhaps the normal "culling" that the game does to remove bloat? There are only so many sims allowed in the world at a time or else the file becomes too large, so over time as new sims enter, some are culled out. I'm on generation 45 of a legacy save and all the siblings a few generations back have dropped off the tree. I can't even get to the top of the tree anymore without the family tree window getting stuck in a "loading" loop, since it's grown so large. I know that there are modded ways to mark sims from getting culled, but apparently doing that can slow the game so I haven't tried it.

Like another person has mentioned, the game culls "unnecessary" Sims, but in your game, if you are able to summon the ghost from the grave still, that hasn't happened yet.

I wish there were a way to cull the sims from the game, but leave the info on the family tree. I've wanted a genealogy update for a long time, including the game's recognition of great-grandparents, adopted sims, vampire spawns, etc. The tree should remain intact, even if the sims are gone and no longer playable, imho, however, I have no idea how or if it is even possible for the dev team to implement.

And that's the problem: the grave is in the cemetery, the Sim's name is written, as is the epitaph and mourning interaction. Despite all, it's as if that sim never existed in the game. All relationships with its children have disappeared, every item it built doesn't have its name on it, and all its heirlooms are completely gone after the will reading.