I think you might be already see this video for BNO055 calibration.
Even we support the fast magnetic calibraion, the planar rotation is not able to achieve the calibration level 3 from level 0.
How did you make the calibration? if the major magnetic distortion is not coming from the lake, you can try to calibrate the sensor nearby the final position to make the fusion lib know about the magnetic environment.
In the other word, when you do the system design, you should keep the magnetic sensor away from the magnetic material as well as the big current lines.