OnDisappearing not called by screen off

Hello,
I have a problem with the OnDisappearing()-method in the ContentPage. The method will not called if I pressed the powerbutton and the app should be in "sleepmode".
The method will called if I press the back-button. For this event it works.

Do you have the bug also?

protected override void OnDisappearing()
 {
       base.OnAppearing();
       camera.DisableCamera();
 }