Hey, I have some info that might help, I'll try my best to explain with my broken english.
I'm having the same issue (no cc/no mods) and I've noticed that this "random sims" on my family tree are actually copies of existing sims. I'm playing the 100 babies challenge, so a lot of my main sim's children have had children of their own and some are already dead of old age.
What I did was to click on the "random sim's" child to open up that family tree, and I've noticed that the "random sim" was actually a copy of my main sim's child, that already show's up on the main family tree. I took some prints to illustrate since I'm not sure you guys will understand what I'm trying to say:
