2019 Sims 4 CAS Sims not saving to Library!

Mods are at risk of breaking with every patch we get, so they need to be checked and the code verified up to date each time you patch your game. We just got a large update in November that changed the Python version for Sims 4, so for sure all script mods made from before November 2018 that haven't been updated are definitely not any good.

Custom content can cause crazy issues too (trust me, I know from learning this hard the way), so you should really test with all mods and custom content removed. Then put things back in small batches until you find whatever is triggering the problem and remove it.

You should also delete your localthumbcache file too whenever you add or remove mods, but I do it at the end of every play session to be on the safe side.