Mongolian and Tibetan Ethnic Identity Certification Act Article 6

Should a Mongolian or Tibetan individual obtain certification recognizing his or her identity as a person of any other special ethnic group, he or she shall forfeit his or her Mongolian or Tibetan ethnic identity.
中文原文 蒙藏族身分證明條例第6條
蒙藏族取得其他特殊族籍身分證明者,喪失蒙藏族身分。

English translation for reference only; the Chinese text prevails. Not legal advice.