The Sims 4 team are all humans and bug fixes are worked on by humans. As with any game, bug fixes have the chance of causing issues elsewhere in the games code and sometimes it is not clear why or what has caused that new issue. The Sims 4 is also a game that is played so differently by millions of players that so many player may experience a bug that someone else has never experienced. Major patch updates change a lot of the coding to the game and this sometimes has a ripple effect in other areas of the game. It's why bug reporting is very important because quite often it might be a bug that no one else has had before. The Sims team has a very large QA team by all accounts and does not use AI to fix bugs.