When she was first brought to South City, Yun Xinyan stayed in the New World Building and never left it, so she naturally did not know about the existence of the Underground City in South City.
After being abducted for the second time, Yun Xinyan was always in Dong Mei's Different Space, and with her impending childbirth, her energy was limited. Therefore, even if she was eventually brought to the Underground City in South City for labor, Yun Xinyan was unaware.
When Ye Tian fought Elder Nan Feng, Yun Xinyan was abducted by Elder Dong Feng's personal guard, Vermilion Bird. However, by then, she had already fallen into a coma and was entirely unaware of the Underground City's existence in South City!
This is precisely why Yun Xinyan feels unfamiliar with the Underground City in South City now. She was merely a pawn in the hands of the Black Hole Clan and never truly understood the city of South City...
