Shift+Tab not working in TreeView control

Question

I cannot get backwards navigation using Shift+Tab to work in a TreeView that contains TextBoxs, forward navigation using Tab works fine and jump from TextBox to TextBox inside the TreeView. Anytime Shift+Tab is used when one of the TextBoxes inside the TreeView, then the focus is move to the previous control outside the TreeView, instead of the previous control inside the TreeView.

Also its only Shift+Tab navigation that are not working correctly, Ctrl+Shift+Tab work as expected and in the correct order.

Any suggestions to what I'm doing wrong?

Example code:

<Window x:Class="TestTabTreeView.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525">

<Window.Resources>
    <Style TargetType="TreeViewItem">
        <Setter Property="KeyboardNavigation.TabNavigation" Value="Continue" />
    </Style>
</Window.Resources>

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

    <TextBox Text="First Line" Grid.Row="0" />

    <TreeView Grid.Row="1" KeyboardNavigation.TabNavigation="Continue" IsTabStop="False">           
        <TreeViewItem IsExpanded="True"><TreeViewItem.Header><TextBox Text="Popular Words"/></TreeViewItem.Header>
            <TreeViewItem><TreeViewItem.Header><TextBox Text="Foo"/></TreeViewItem.Header></TreeViewItem>
            <TreeViewItem><TreeViewItem.Header><TextBox Text="Bar"/></TreeViewItem.Header></TreeViewItem>
            <TreeViewItem><TreeViewItem.Header><TextBox Text="Hello"/></TreeViewItem.Header></TreeViewItem>
        </TreeViewItem>
        <TreeViewItem IsExpanded="True"><TreeViewItem.Header><TextBox Text="Unpopular Words"/></TreeViewItem.Header>
            <TreeViewItem><TreeViewItem.Header><TextBox Text="Work"/></TreeViewItem.Header></TreeViewItem>
            <TreeViewItem><TreeViewItem.Header><TextBox Text="Duplication"/></TreeViewItem.Header></TreeViewItem>
        </TreeViewItem>
    </TreeView>

    <TextBox Text="Last Line" Grid.Row="2" />
</Grid>