August 2020
I noticed after my sims returned from school that one of my sims, a teenager, was deleted from my household, and I believe from existence. She didn't die because there would've been a notification, and my sims would've gotten moodlets. She also was not in my family tree, and none of my sims know her or have a relationship with her. I do have more than eight sims in my household but have a mod installed to allow more sims than the game allows.
August 2020 - last edited August 2020
@myqueenrocky There may be an issue with the Mod, and it could be it needs an update if this happened after a patch and not before. I would recommend that you:
Now enter the game and see if the issue re-appears.
If that doesn't fix it , the issue may be caused by a different Mod or CC. To find out which one, please try the following in this order without skipping any steps, even if you’ve done some of these steps already:
Do not re-add any of your Mods or CC at this time. Please then restart your game and do not enter your previous game-save but begin a new game. See if the issue is present in this new game-save.
If this fixes the issue you can re-add your Mods/CC one at a time to see which were conflicting with the function/causing the glitch - but do not re-add the old Resource.cfg file from the old Mods folder, as it is possible for it to have become corrupted. If you have a lot of Mods or CC you can do so by via the 50/50 method.
August 2020
@myqueenrocky Please note this information added to point # 4 above:
Make sure when you split the household that you split them in "Household Manager", not in CAS. If you open the household in CAS and it has more than 8 the game will delete some of the sims until there are only 8.
Also, due to this dynamic, (the game deleting extra Sims from a household when opened in CAS if it totals over 8), is there a chance that you opened CAS while the teen was at school? If so, this may have triggered her being deleted at that time. (Thank you for pointing this fact out @EuphorialQueen )