By default the Back key will terminate Qt for Android apps, unless the key event is accepted. Unfortunately, due to misleading documentation (QTBUG-35155), many applications (including Qt examples) are incorrect, and will block the back key. We recommend making the logic explicit: always accept() the Key_Back event, and quit() the application explicitly if required.
According to https://wiki.qt.io/Qt_for_Android_known_issues:
Which behavior do we want for the back key?