Drifting compass heading after using new Onboard Calibration

I am using Mission Planner 1.3.56 to do Onboard Calibration (new method) for the externally mounted HMC5983 compass installed on my Align 450 flybar heli. FC is Pixhack flashed with Copter 3.5.5 heli.
Every time after calibration, the message says “Mag cal successful”. After reboot and connected to MP again, the heading (on top of HUD) in MP drifts, and I consider drifting heading means calibration is not properly done even though the heli is stationary and the positive message.
I have been using the old Offboard Calibration method to calibrate a 450 size quad with Pixhawk, and another flybarless 450 heli with APM 2.6, all without problem…as soon as you stop rotating the vehicle, the heading on top of HUD stops and lock on a certain heading.

After many failures at home, I did the calibration in a park, away from metal fence, and the result is the same…drifting heading.
I confirm the compass mounting orientation is correct …roll 180 deg and facing forward.I also tried to enable or disable Compass #2 (internal compass) before and after calibration, and tried to enable only Compass #2 and disable Compass #1 (external). All these do not change the result.
Pls share any experience you may have on this new method.