The "distance" thing still makes no sense. As I said, it measures the field vector (strength and direction) on startup - it doesn't need a calibration to do that. And it calculates declination - a calibration simply cannot determine that. The only thing the calibration is for is to subtract out the magnetic field of the aircraft components themselves, and those don't change with distance or, generally, with time, only by adding or changing components or exposing the aircraft to a strong magnetic field.
Whether it makes sense to you or not my drone was doing it based on distance which was reflected clearly in the .dat flight logs.
So someone at DJI engineering clearly thought it was needed at the time (but gone since the last update).