I found ecl has been changed since the last commit here https://github.com/PX4/Firmware/commit/6c02cc53deec917b58b4728cdf20b0f99d5a3848
flowSample has member Vector2f flowRadXYcomp
And this variable is updated in estimator_interface.cpp when new flow data is given
But in the latest master I found this variable is moved to src/lib/ecl/EKF/ekf.h
And it looks like flow rate compensation is done in control.h as shown here
Why flow rad compensation has changed and how to modify pwm3901 can work again?