Possible issue with focusing

[Alexus]

There is an window with three combo boxes:

<Window ... >
    <StackPanel>
        <telerik:RadAutoCompleteBox/>

        <telerik:RadAutoCompleteBox
            x:Name="_combo"
        />

        <telerik:RadAutoCompleteBox/>
    </StackPanel>
</Window>

and back code:

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        Loaded += MainWindow_Loaded;
    }

    void MainWindow_Loaded(object sender, RoutedEventArgs e)
    {
        _combo.Focus();
        //_combo.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next)); //(!)
    }
}

Steps:
1. show window
2. type some text (without tabs)

expected: some text appears in second combo box with name "_combo".
actual: second combo box has focused appearance, but does not accept any input
actual with line (!) uncommented: as expected.

This is strange behavior because, for example, dateTimePicker would work as expected without uncommenting line (!)