We are currently doing the test by manually moving the boat. The calibration status of the magnetometer drops only if we move it. If we keep it in one position without moving, the calibration doesn't change.
This is the plot of the magnetometer from status 3 of calibration to 1. The violet line is the calibration status (from 30 = 3 to 10=1). The time on X axis is in milliseconds
From the plotter, there is quite big magnetic value change.
When magnetic environment changes too fast / too big, then the calibratio status will drop from 3 to 2 or even 0. this will let system know that you will need a new calibration movement to bring back the calibration level to 3 again.
When calibration level dropped, additional offset found in each axis which will definately cause a heading drift.