but I it is running Arducopter
I see, but that’s exactly the problem we’ve hit earlier (and the whole point of this thread) - we’ve had experiences when the magnetometer on the Proficnc / Hex GPS module was not used by PX4 because there was no driver for such a magnetometer (in our case ICM20948) in the firmware. However, for most people it “works”, since they run Arducopter and apparently there is such a driver in Arducopter firmware.