Parent dependency property is not updating child dependency property using Dispatcher.Invoke - wpf

Question

I have two user controls
parent Orders that creates a list of anonymous objects
child Table that shows this list in a DataGrid
Parent XAML and CS
public partial class OrdersControl : UserControl
{
public static readonly DependencyProperty OrderItemsProp = DependencyProperty.Register(
"OrderItems",
typeof(IEnumerable<dynamic>),
typeof(TableControl),
new PropertyMetadata(null));
public IEnumerable<dynamic> OrderItems
{
get { return (IEnumerable<dynamic>)GetValue(OrderItemsProp); }
set { SetValue(OrderItemsProp, value); }
}
}
...
<UserControl x:Name="OrdersContainer">
<ctrl:TableControl x:Name="ChildTable" Items="{ Binding OrderItems, ElementName=OrdersContainer }" />
</UserControl>
Child XAML and CS
public partial class TableControl : UserControl
{
public static readonly DependencyProperty ItemsProp = DependencyProperty.Register(
"Items",
typeof(IEnumerable<dynamic>),
typeof(TableControl),
new PropertyMetadata(null));
public IEnumerable<dynamic> Items
{
get { return (IEnumerable<dynamic>)GetValue(ItemsProp); }
set { SetValue(ItemsProp, value); }
}
}
...
<UserControl x:Name="TableContainer">
<DataGrid
ColumnWidth="*"
x:Name="DataItems"
IsReadOnly="True"
CanUserAddRows="False"
VerticalAlignment="Top"
GridLinesVisibility="Vertical"
AutoGenerateColumns="True"
CanUserDeleteRows="False"
HeadersVisibility="Column"
ItemsSource="{ Binding Items, ElementName=TableContainer }"
MinColumnWidth="0">
</DataGrid>
</UserControl>
Scenarios
If I initialize parent property OrderItems in the parent constructor, it works fine, so data binding works
public OrdersControl
{
OrderItems = new[]
{
new { Name = "A", Price = 1 },
new { Name = "B", Price = 2 }
};
InitializeComponent();
}
If I subscribe to updates from IObservable and update OrderItems property, I get a UI thread exception
If I wrap initialization of OrderItems with Dispatcher.Invoke, nothing happens, child DataGrid stays empty
accounts
.Select(account => account.Orders.Stream)
.Merge()
.Subscribe(message =>
{
Application
.Current
.Dispatcher
.BeginInvoke(new Action(() => OrderItems = items));
});
Why does child control is not initialized if initialization is inside Dispatcher call?

Answer 1

For now, I decided to set child DataGrid.ItemsSource property directly from parent code-behind. Still can't explain why setting parent dependency property doesn't initialize child dependency property with Binding in XAML.
Parent CS
accounts
.Select(account => account.Orders.Stream)
.Merge()
.Subscribe(message =>
{
Application
.Current
.Dispatcher
.BeginInvoke(new Action(() => ChildTable.DataItems.ItemsSource = items));
});