Something interesting … In OsmAnd’s (yes, Android, but hang on) Settings - General settings you can toggle “Use magnetic sensor (For the compass reading, use the magnetic sensor instead of the orientation sensor)” on and off.
I just tested that briefly, and choosing the magnetic sensor here seems less unstable to me. Perhaps Sailfish OS on the Fairphone 2 could somehow mirror this setting and let users choose the sensor?
(I’m aware this is not a general Android setting, it’s in the App, and the App is Open Source.)