How to create a re-useable ContentView as a base view?

In my .NET 9 MAUI project, I like to have a base ContentView to reuse for multiple pages. For that, I created a ContentView like this:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage
	x:Class="LanguageInUse.Views.BaseAdvView"
	xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
	xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml">

	<Grid>
		<Grid.RowDefinitions>
			<RowDefinition Height="*" />
			<RowDefinition Height="Auto" />
		</Grid.RowDefinitions>

		<ScrollView>
			<ContentView x:Name="ContentPlaceholder" />
		</ScrollView>

		<Label
			Grid.Row="1"
			Margin="10"
			FontSize="Medium"
			HorizontalOptions="Center"
			Text="Advertisement"
			TextColor="Black"
			VerticalOptions="Center" />
	</Grid>
</ContentPage>

with this code behind:

public partial class BaseAdvView : ContentPage
{
    Advertisement? adv;

    public BaseAdvView(Advertisement advertisement)
    {
	    InitializeComponent();

	    adv = advertisement;
    }

    public void SetContent(View content)
    {
        ContentPlaceholder.Content = content;
    }
}

When I derive a new page from it, I have this XAML:

<?xml version="1.0" encoding="utf-8" ?>
<bav:BaseAdvView
	x:Class="LanguageInUse.Views.MainPage"
	xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
	xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
	xmlns:bav="clr-namespace:LanguageInUse.Views">

	<ContentView>
		<VerticalStackLayout Padding="30,0" Spacing="25">
			<Image
				Aspect="AspectFit"
				HeightRequest="185"
				Source="dotnet_bot.png" />
		</VerticalStackLayout>
	</ContentView>
</bav:BaseAdvView>

and the code behind is:

public partial class MainPage : BaseAdvView
{
    private readonly MainPageViewModel? vm;
    private readonly Advertisement adv;
    private readonly UserSettings settings;

    int count = 0;

    public MainPage(MainPageViewModel model, Advertisement advertisement) 
        : base(advertisement)
    {
        InitializeComponent();
        SetContent(Content);
    }
}

The result is that the content from the derive page is shown but the Label in the base page doesn't.

What is the correct way to create a base page?