I was once a great fan of xiaomi. I bought my mi4 which seems to be the best phone under 15000rs but then after using it for 2 months. This annoying problem occurred.
The navigation buttons responded abruptly first the home button started opening recent app tray and then in a few hours the back button also started opening recent app tray and after further use stopped working anymore.
It was not only me with whom this problem occurred. There are many users who face this similar situation in mi forum and the worst part is that they have no solution.