When Megan Olson lands in South Korea for the Winter Olympics next week, she'll feel something that is both surreal and vivid.

An intoxicating sense of belonging.

A deep sense of loss.

Pride, for the motherland she barely knows after being secretly adopted away.

The 33-year-old social worker from Minnesota is joining dozens of fellow South Korean adoptees who are returning to their birth country for the 2018 Winter Games in Pyeongchang.

Many endured cultural, racial and national identity issues stemming from an international adoption phenomenon that peaked in the 1980s. Now, the once-in-a-life...