UWP : FillAndExpand expand out of screen of a modal content page

In a modal content page, I have a grid :

<Grid HorizontalOptions="FillAndExpand" VerticalOptions="FillAndExpand"> <Grid.RowDefinitions> <RowDefinition Height="*" /> <RowDefinition Height="40" /> <RowDefinition Height="40" /> </Grid.RowDefinitions> <StackLayout Spacing="0" VerticalOptions="Start" HorizontalOptions="FillAndExpand" Orientation="Vertical"> <2 LABELS /> </StackLayout> <Button Grid.Row="1" /> <Button x:Name="BadButton" Grid.Row="2" /> </Grid>

The Button called BadButton is pushing out of the screen or is barely visible at the bottom of the screen.
When I checked the size of the MainPage, it says 616px. The Grid size is 640px !

What I am doing wrong ? Is it a bug of Xamarin Forms ?