I have seen a plethora of gadgets for smartphones that implement a magnet of some sort. For example, I've seen a number of automobile mounts that involved a powerful but small magnet to stabilize the phone in the cradle.
Just know that when you expose your smartphone to such a magnetic force, you totally screw up the internal electronic compass. The messed up compass calibration will manifest itself in some irritating or annoying ways when you are using any GPS app for automobile navigation such as WAZE or Google Maps, etc as well as geocaching apps such as C:GEO or Cachly, etc. The effect is that while you're standing still, on foot or in the car, the phone has no way to accurately tell the software which way you are facing. So it either guesses erroneously or simply wigs out bouncing all over the place.
To be honest, I'm not sure that there is any permanent damage to the internal compass, but I do know it sends the calibration out of whack. Recalibration depends on the device, the operating system, and the app(s) used. But if your phone is acting strange sometimes, consider any scenarios where it may be in close proximity with a magnetic force.
Even without the aid of a compass, the smartphone's gps functions are unaffected as long as it can "see" satellites and you are moving. While moving your device can accurately detect your position, bearing, and course over ground.
Anyway, I just thought I'd pop up and toss that out there in case it solves any mysterious phone app behavior for some fellow catchers.