WPF Textbox TwoWay binding in datatemplate not updating the source even on LostFocus

I have an ObservableCollection<string> Tags as part of a custom object. I bind it to a DataTemplate in order to show all tags to the user with the following code:

<StackPanel DockPanel.Dock="Top" Margin="15,0,15,0" Orientation="Horizontal">
    <Label Content="Tags:" FontSize="14" Foreground="{StaticResource HM2LightTextBrush}"/>
    <Grid>
        <ItemsControl Name="PanelPreviewNoteTags" ItemsSource="{Binding ElementName=lbNotesQuickView, Path=SelectedItem.Tags}" Margin="3,0" Visibility="Collapsed">
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <WrapPanel/>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <Border BorderThickness="1" BorderBrush="#676B6E" Margin="3,0">
                        <Label Content="{Binding .,Mode=OneWay}" Foreground="{StaticResource HM2LightTextBrush}"/>
                    </Border>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
        <ItemsControl Name="PanelEditNoteTags" ItemsSource="{Binding ElementName=lbNotesQuickView, Path=SelectedItem.Tags}" Margin="3,0" Visibility="Collapsed">
            <ItemsControl.ItemsPanel>
                <ItemsPanelTemplate>
                    <WrapPanel/>
                </ItemsPanelTemplate>
            </ItemsControl.ItemsPanel>
            <ItemsControl.ItemTemplate>
                <DataTemplate>
                    <Border BorderThickness="1" BorderBrush="#676B6E" Margin="3,0">
                        <StackPanel Orientation="Horizontal">
                            <TextBox Text="{Binding ., Mode=TwoWay}"/>
                            <Button Style="{StaticResource RibbonButton}" Click="ButtonRemoveTagClick" Tag="{Binding}">
                                <Image Height="16" Width="16" Source="/Poker Assistant;component/Resources/fileclose.png" />
                            </Button>
                        </StackPanel>
                    </Border>
                </DataTemplate>
            </ItemsControl.ItemTemplate>
        </ItemsControl>
    </Grid>
</StackPanel>

Adding and removing items from the ObservableCollection works as expected.

In code I switch between edit and view mode by setting the Visibility of the corresponding PanelEditNoteTags and PanelPreviewNoteTags. This all good and working. But when I enter the edit mode and start typing new values for the tags in the TextBox the source doesn't get updated. I certainly know that the LostFocus event is raised when I press my Save button. I tried all UpdateSourceTrigger values, still the same.

Is it a problem related to two controls binding at the same time to the same value - the Label from PanelPreviewNoteTags and the TextBox from PanelEditNoteTags?

What am I missing here?