When you add FormsAppCompatActivity the OnDisappearing method is called when the device sleeps, but OnAppearing is not called on device resume. If you use the normal FormsApplicationActivity OnDisappearing is not called when the device sleeps, which is how it should be.
Here is a Google Drive link to an example project. If you just run the project, click page 3, then watch the debug log. When you sleep the device you'll see the message "ON DISAPPEARING!!!", this should only happen when you leave the page.
Note: This is a huge issue since it breaks any app using FormsAppCompatActivity. Please fix as soon as possible. I have to delay my app because of this.
Confirmed - the behavior is not consistent between FormsApplicationActivity and FormsAppCompatActivity.
This seems to be fixed in Xamarin.Forms 2.2
In XF 2.2 The OnAppering/Dissapearing is extremely broken. Every page in you navigation stack will trigger onappering when coming back from background. And every page will also trigger dissapearing when entering background, Or changing activity (for example taking picture with Xam.Plugin.Media)..
Should be fixed in 2.3.4-pre1. Please note that you will need to opt in to the fix.
See https://github.com/xamarin/Xamarin.Forms/pull/450 for details. Thank you!