In the guides for "LayoutOptions": developer.xamarin.com/guides/xamarin-forms/user-interface/layouts/layout-options/#Alignment , I read this:
A StackLayout does not respect the Start, Center, End, and Fill LayoutOptions fields on child views that are in the same direction as the StackLayout orientation. Therefore, a vertically oriented StackLayout ignores the Start, Center, End, or Fill fields if they are set on the VerticalOptions properties of child views. Similarly, a horizontally oriented StackLayout ignores the Start, Center, End, or Fill fields if they are set on the HorizontalOptions properties of child views.
I don't understand what does "respect" mean here, I actually can change the VerticalOptions of a view inside a StackLayout of Orientation set to "Vertical" in a normal way, and I always make use of it!