This is a bit OT and anecdotal but I have seen a Korean-Chinese (joseonjok) waitress treated like total crap verbally by her colleagues in a Korean restaurant, in front of dining guests. Of course I couldn't understand what was said since it was in Korean but you could tell just from body language. When she came to our table to take the order she switched over to mandarin when she saw we were Chinese, and maybe felt more comfortable with it. I think the S Koreans don't consider them real Koreans and the Chaoxian zu themselves probably identify more as Chinese than Korean.