XAML gradient issue in UWP for some devices
Solution 1:
Try removing the Grid.RowSpan="2" from the Rectangle (or add a RowDefinition), you have 4 rows (4 RowDefinition) but with the Rectangle having Grid.RowSpan=2 it adds up to 5 rows, so it might be causing you trouble.
EDIT: My bad, Rectangle actually spans over rows 2 and 3 (Grid.Row="2"), so it's ok.
Since you're just stacking <StackPanel> over <Grid> (with nothing fancy), you could try to replace the root layout <Grid x:Name="LayoutRoot"> with <Canvas x:Name="LayoutRoot"> and see if that makes a difference.