So I have this female young adult sim, the first thing I do when I move her into sunset valley after creating her is sending her on a 3 day trip to China...

So during her first trip to China, she did a few adventures & while doing them she met this elderly man that she had to talk to for an adventure(she never saw him again after the adventure was over)

After she went back home the old man was on her list of friends(listed as a stranger) & a few days later his icon vanished from it..

So just recently I sent her back to China for a second trip so that she can master the Martial Arts skill, and as I'm playing right now her last name has mysteriously been changed from Andrews(her CAS name) to Hong...

I decided to check her family tree just a few minutes ago & I see that it shows her having a child in the child stage of life, I go to see who the father is & it is that same old man from earlier in the post... but now his icon is grayed out which means he passed away

What I don't understand is that I made my sim straight out of CAS as a single sim with NO family members, & I've been controlling her this entire time... I never switched households or turned on free will(actually it has been off the entire time I've been playing her)

So there is no way she could have had some kind of relationship with this old man, as he never became anything more to her then a stranger...

I checked her scrapbook & she has no memories of ever interacting with him again after the adventure where she was required to..

She also has no memories of having/adopting a child & nothing that would explain her last name being changed from Andrews to Hong...

I have also been unable to locate this "child" that is supposed to be her child, it says that he lives in china but I cannot find him anywhere...

Did your sim ever enter a time machine? Otherwise I have no idea how this could have happened unless there was another sim in china with the same name as your sim.
If you want this fixed, the best way would be to use MasterController by NRaas, its a mod, but once in game you click on the townhall, nrass, basic, sim, total annihilation then choose the name of the old man and the child and both will be deleted completely from your save file, and from the family tree. then you click on your sim, basic, edit in CAS then change her last name

I think that is a known bug that Pescado set up preventions for in AM. Foreign sims who die need to be replaced by children. So they marry and have children with the sim they have the highest relationship with. And if your sim is the one that has interacted with them the most, guess who gets to be married and have children? Even if you were already married at home.