Disappearing Sims

Update

The bug part of this was fixed.

Culling isn’t a bug but a performance mechanism.

After switching to another family, most of my Sims of the other family have disappeared. They are not dead but simply not there anymore.

My Sim has made quite a few friends around town but the next day they have all disappeared.

The game purges Sims that are distantly related to your Sim every Wednesday to Thursday Night. There is a bug in the game that needs fixing, since played Sims are also being purged. At this point it’s unclear what the intention of the purge is and which Sims should indeed be purged and which should not.
The game has a max. population. If you play a save game for quite some time, NPCs that aren’t saved in My Households are being culled. More Info can be found here: Culling – What is it and what does it do ?

This issue should be fixed
Go to Manage World and make sure that all Sims you want to keep are in My Households