Sim was unwillingly deleted from household

[myqueenrocky]

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. 

[SheriGR]

@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:

1. Roll your game back to a game save from before the teen went missing. Directions on how to do this:
   https://sims4.crinrict.com/eng/2014/09/save-games/
2. Save a backup of your game at that point (directions for doing this are at the above link also).
3. Now check with the Mod maker and see if there is a new, updated version. If there is you could try adding that and see if the issue is fixed. If there's not a fix, you could do the following:
4. Then I would split the households into households of 8 or less. Save them to your household manager so it preserves their relationships. (If you save to the library the only relationships that are preserved are those in the household they're saved to he library in.)
   Edit: 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. (Thank you for pointing this fact out @EuphorialQueen )
5. Then remove the Mod from the game, plus also remove the localthumbnailcache.package file in your “The Sims 4” folder.

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:

• Exit your game then exit Origin. Drag the Mods folder from your 'The Sims 4' folder onto your desktop (for future reference) then delete the localthumbcache.package file in your 'The Sims 4' folder. (When you restart the game it will automatically create a new Mods folder.)
• Next please run a game repair [ Open Origin, click 'Game Library' then right-click Sims 4 > Repair Game ].
  * If you’re on PC, make sure to let it run until it has completed all repairs (it will do the base game first, then run through each of your expansions/packs).
  * If you’re on Mac, the repair in Origin will only repair your base game.

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.

[SheriGR]

@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 )