I'm fairly confident this is a design problem, there are numerous reports on the Asus website of this problem even after the display has been changed. It generally seems to occur after a few months of use, there is a reddit post which claims its caused by bending, the resolution is to put the phone face down and apply pressure on the back cover, this does seem to resolve the problem but only temporarily. Asus doesn't appear to have any intention of commenting on any type of fix, I'm suprised its not been picked up AC but I have no intention of buying another Asus phone anyway.