Originally Posted by
wphamilton
A magnet will not interfere with the display itself, nor the any of the phone electronics except if it has a compass/magnetic field sensor.
In your case the LG G3 probably has the magnetic sleep feature, a tiny sensor that puts the phone to sleep when the case is closed and the magnet was close enough to confuse it.
I know that the LG G3 phones touted some special cases with a circular window. Your explanation sounds very likely correct.