Ideas
I've only noticed this in 3 specific worlds: Sulani and San Myshuno (I voted in those threads too), and now in the new world, Gibbi Point. Gibbi Point is the weirdest, because they're not just coming to drink water and leave like in Sulani, or cutting through my apartment like it's part of the street in San Myshuno — in Gibbi Point the Sims act as if I invited them over when I didn't. Random Sims, some of which my Sims do know, others are total strangers, are just fully in my house using all my stuff, butting into conversations or distracting my Sims, and there's no option to send them home, I just have to wait for them to leave on their own. I even have the Private Dwelling lot trait on, but the only way to prevent them from breaking and entering is to lock all the entrances. I do have some mods, but the biggest one is lumpinou's RPO mod, the rest are mostly script mods like disabling burnout and making Sims use their phone less, and some trait enhancements, so I'm not sure if they'd be the culprit here? But I also haven't seen anybody else mention Gibbi Point yet so ¯\_(ツ)_/¯