MPU-9250 compass always returns to zero
Ok, I have finally found the issue, and I am feeling very, very stupid.


I needed to calibrate the magnetometer with the figure-8 movement
I put a gigantic solid-metal 18650 Li-ion cell right next to the magnetometer chip...


EDIT: after replacing the 18650 with a polymer battery that is far away from the chip, things got a little better but both sensors in both devices still return garbage values.