Why Xamarin form App turn to "Sleep" after I drop down the iOS notification center

My program is based on Xamarin form.
Each time, both in real phone or simulator, after I drop down the iOS notification center. The "OnSleep" method was called in App class.
After I push up the notification center, the App still freeze, until after about 20 seconds, the "OnResume" was called in App class.

So why cause this? And how to fix this, let the program can work immediately after the notification center is pushed back.

Thanks