First only a couple lots randomly disappeared from my neighborhood, restoring a backup did not work. Suddenly more lots disappeared. I don't know why this happens and how to stop this. I do not know their lot numbers. The only fix I can find is the one where you put them in the lot bin in via the sims 2 folder but there's no point in doing that if they will disappear again.

Do you have an antivirus or OneDrive sync enabled? That could mess with the game files.

Could be but I haven‘t changed anything with both of those programs for months and nothing happened until a few weeks ago

Personally, I find them unpredictable, because they do stuff automatically, so that should be the first thing you check.

Sometimes a Windows update will reset your settings for One Drive I've noticed.