Kivy application on Android goes black after screen unlock

The on_pause() and on_resume() methods of the main App class have been implemented as follows:

class FitnessApp(App):
    def build(self):
        return RootWidget()

    def on_pause(self):
        return True

    def on_resume(self):
        pass

Logcat output relating to the app from screen lock to unlock

06-15 22:16:35.494  5318  5613 D PowerCheckerService: foreground, uid = 10306, app =  org.workouthelper.workouthelper
06-15 22:16:39.038  5318  5613 D PowerCheckerService: foreground, uid = 10306, app = org.workouthelper.workouthelper
06-15 22:16:39.371  2052  5221 I Timeline: Timeline: Activity_windows_visible id: ActivityRecord{18c71e u0 org.workouthelper.workouthelper/org.kivy.android.PythonActivity t9694} time:49848452
06-15 22:16:39.373   904  1349 E BufferQueueProducer: [SurfaceView - org.workouthelper.workouthelper/org.kivy.android.PythonActivity#0] query: BufferQueue has been abandoned
06-15 22:16:39.373   904  1349 E BufferQueueProducer: [SurfaceView - org.workouthelper.workouthelper/org.kivy.android.PythonActivity#0] dequeueBuffer: BufferQueue has been abandoned
06-15 22:16:39.373   904  1349 E BufferQueueProducer: [SurfaceView - org.workouthelper.workouthelper/org.kivy.android.PythonActivity#0] query: BufferQueue has been abandoned
06-15 22:16:39.373   904  1349 E BufferQueueProducer: [SurfaceView - org.workouthelper.workouthelper/org.kivy.android.PythonActivity#0] dequeueBuffer: BufferQueue has been abandoned

Weirdly enough the app goes back to normal after opening the notification tray and closing it again.

Version info

1. Android 10 QKQ1
2. Python 3.7.6
3. Kivy 1.11.1
4. Buildozer 1.2.0

Any help on this would be greatly appreciated!