How are Korean adoptees seen by Korean society?

I work with multiple Korean adoptees and Korean immigrants in my workplace, this is purely based on my personal experience. 
To answer your question, Korean adoptee/trans-racial adoption is a quite unfamiliar identity in Korean society. I rarely met one in Korea. In my work place (in the U.S.), the adoptees are usually viewed as foreigners by Korean immigrants, even though we look alike. If the adoptee speaks Korean fluently, then it may increase their sense of belonging in the Korean society. I personally think if the adoptees identifies themselves as Korean, that should be honored. 
I would say adoption in general is a huge taboo topic in Korea. People normalize nuclear family and consider others as strange or outliers, there is a lot of stigma attached to different forms of family in Korea . Divorce or remarrying is also kept private and personal. We don’t publicly talk about step- or half- family members either.