Call me old fashioned but I still prefer 3 button navigation. I experience this problem intermittly, usually when an app becomes slow or unresponsive, pressing the previous apps key, the application preview becomes transparent then pressing the home key then pressing the previous apps key again, the previous apps key becomes unresponsive.
I'm using KISS launcher but changing to stock launcher doesn't change this behaviour.
I can restore function to the previous apps key by going to settings, switching to gesture navigation, swiping up to show previous apps, tapping back to settings then switching back to 3 button navigation.