The treaty says: "An individual who is, or immediately before visiting a Contracting State was, a resident of the other Contracting State and is temporarily present in the first-mentioned Contracting State for the primary purpose of teaching, giving lectures or conducting research at a university, college, school or other accredited educational institution or scientific research institution in the first-mentioned Contracting State shall be exempt from tax in the first-mentioned Contracting State for a period not exceeding three years in the aggregate in respect of remuneration for such teaching, lectures or research."
The treaty does not say visa status, does not say "go directly from China, not other countries," The conditions are only (1) Resident of China; (2) teaching or research in university, college, school or other accredited educational institution or scientific research institution. No any other conditions are specified.
No matter how many you go directly from China or other countries, as long as you still a "Resident of China", you are eligible for the tax benefit.