Compass off by full quadrant(s) (90/180/270) until calibrated
This is rather a warning to other Locus Map users. I noticed that when I display the Compass screen, the compass is often off by 90 / 180 / 270 degrees (differs based on the orientation of my phone's display).
This happens quite regularly after not using Locus for a few days. You can identify such a case by simply rotating your device so that the display orientation is changed. In this case, you get compass reading where the North points to different direction on each screen (in my case, to West when holding the phone horizontal in 'portrait' mode, to South in one of the 'landscape' orientations and to North only in the second 'landscape' orientation of display).
After you move the device for a while around all axes, the calibration takes places and since that moment, the North points in the same (correct) direction whatever orientation your display is in.
Be aware of this problem, because one can't always quickly figure out his compass is in trouble (and you may be soon then too)!
I also contacted Locus development team, but they said they can't fix it / can't figure out the compass needs calibration and they also do not want to implement a warning screen requiring calibration when you enter the Compass screen the first time after starting the Locus Map app.