@Anonymous while I generally agree with @KarenJ2 but I would like you still to consider the following:
(a) if when you left the USA in 09/01/2018, your visa was terminated AND you had no intention of returning i.e. you actually became a Non-Resident Alien ( 2018 being a dual status taxpayer status for you ), then that you were a resident alien for tax purposes status was terminated and invalid to consider.
(b) if however, your current employer had applied for H-1B request for you before you left for China and you were in wait period, then @KarenJ2 is correct and you have to consider the Resident Alien for Tax purposes to have continued and you file as she is suggesting .
(c) if above (a) is true then your 2019 status is a dual status i.e. self-employment income in China is not subject to US taxes, just Chinese taxes. Your actual start date is 03/01/2019 and thus your 2019 return is s short year ( 10 months ?)
IMHO
