Entry with numeric/telephone keyboard changing app background color

I'm noticing very odd behavior on Android using Xamarin.Forms. Typing into an Entry with a numeric/telephone keyboard (all other types don't have this issue) darkens the background color. This behavior occurs even on a clean project with nothing but an Entry:

using Xamarin.Forms;

namespace App1
{
    public class App : Application
    {
        public App()
        {
            MainPage = new MainPage();
        }
    }

    public class MainPage : ContentPage
    {
        public MainPage()
        {
            Content = new Entry { Keyboard = Keyboard.Numeric };
        }
    }
}

When the Entry is first focused into, nothing unusual happens. In fact, if you click the tick at the bottom right of the keyboard to close it without entering anything, nothing happens either. But if you type anything (even if you backspace it), the background color of the app changes. It even affects other pages in a navigation stack.

What is the cause of this can how can it be fixed?