Is it a breach of contract to go to Hong Kong to study PhD after graduation from Japan?

No, but you need to write an application to the Beijing Municipal Committee and get approval.

Conditions:

First, I love the motherland and socialism, have good political and professional qualities, have no record of violating laws and regulations, have outstanding performance in work and study, and return to China to serve the motherland after graduation;

Second, it has a good professional foundation and development potential, and its foreign language level meets the requirements stipulated in the Foreign Language Conditions for Overseas Students Funded by the National Fund for Studying Abroad.