URGENT! 1 sim vanishes from household once I press play on the family.

I have put this post up around 12 days ago on a separate forum and got no response, issue is still happening so thought I'd try again.

This has happened with 3 seperate households. the first time I aged up a sim, edited him in CAS, then once i pressed back to live mode, he wasn't there at all and not in the gallery. The second time, it was a child sim in a family I just made and moved into a house, then he vanished. I've just made another family, and the same thing has happened. I have tried recovering the save file, repairing the game, starting a new game, removing any mods and checking for viruses but nothing has solved the issue.