It seems to have worked for me! Before my kinda solution one of my sims would vanish, but now my whole family is still there. I haven't tested it with pets or babies, but so far it seems to have helped. I found this in a reddit post and tested it out yesterday and today. 🙂
Someone on reddit figured out that it is an antivirus issue, you just need to make an exclusion for the sims 2 folders so your program skips over it and doesn't flag it. I'll update if I come across the issue again or any other issues. The evidence is I experienced this multiple times and now I'm able to load into my game without any of my sims vanishing. 😺 Just haven't tested pets or babies yet. If anything, it seems to have done something to help, which is progress.