Hey, I'm experiencing the exact same now. I thought I fixed it by removing a UI "fixer" which overrides the sims ui faces when they make emotions and make it more realistic, but then the glitch occurs into hours of me playing, so it's something that I'm just having to work around.
When I was doing the 50/50 method, I realized that by taking out all of my custom content that related to cas (like hairs, clothes, accessories, teeth, etc) that would make the sims load and change their outfits, and even sims who were invisible populated again. I even played with a new save for like 3+ hours with no custom content on, and the glitch never happened (maybe it would've if I played for longer but I'm not sure), so I figured it has something to do with cas-related cc. That's unplayable for me since I heavily rely on cas-cc to make my sims how I want them, but that's what I figure it is.
I'm seeing a lot more people have this issue in some youtuber's discord so hopefully it gets more attention and we get a patch pretty soon.