Cool observation - not as bad on Android due to a hardware back button, but it still happens.

Or another option to solve it??? Just don't show notifications when the user is "obviously" using their phone. e.g. if there's been any tap in the last 5s, make the noise but don't show the pop-up until the screen has gone untouched for 5s (the user can still swipe down from the top to see it).