I know one Boy who was adopted when he was 2 years old, he had birth certificate and got a Thai Passport issued to leave the country. He has no known family. When he was about 20, they went to BKK and tried to get citizenship back. A lawyer looked into it and they were told, not possible.
As I learned, a Thai who got adopted once, will never be able to get back his Thai citizenship, even if he can provide birth certificate and kids passport!