Coerce items at later stage, as now when programmatically add items to Selected Items and Checked Items. and both collections are set before the ItemsSource, indication for selection and checked is not presented.
Issue:
<telerik:RadTreeView x:Name="treeView"
CheckBoxMode="Recursive" Grid.Row="1"
SelectedItems="{Binding CheckedItems, Mode=TwoWay}"
ItemsSource="{Binding Items}">
Work:
<telerik:RadTreeView x:Name="treeView"
CheckBoxMode="Recursive" Grid.Row="1"
ItemsSource="{Binding Items}"
SelectedItems="{Binding CheckedItems, Mode=TwoWay}">