WPF TextBox.Text doesn't update in another control

It is simple thing that works everywhere instead of TextBox.Text without any reasons. What I need is just to get updated Text property of textbox when DataContext changed. There is a DataGrid and another control behind it to show details about row. So when user click's on row I am obtain data object from grid's row and show its details in this details control. Details control contains just propertyName and value control. I.e. textblock and textbox. ANY property of textblock or textbox are updated on changing DataContext except Text in textbox. I've break my head already. The main problem is textbox won't updated after I've changed it. And it works fine if move textbox outside control to the details control (parent)

The property-Value control:

    <DockPanel x:Class="UserControls.PropertySectionControl"
                 xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
                 xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                 xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
                 xmlns:d="http://schemas.microsoft.com/expression/blend/2008"              
                 mc:Ignorable="d"       
                 x:Name="_propertyDockPanel"
                 HorizontalAlignment="Stretch"  Margin="0 5 0 0"
                 d:DesignHeight="300" d:DesignWidth="300">

        <TextBlock  Text="{Binding Path=Property, ElementName=_propertyDockPanel}" TextWrapping="Wrap"/>
        <TextBox              
  Text="{Binding Path=Value, ElementName=_propertyDockPanel, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                 IsReadOnly="{Binding Path=IsReadOnly, ElementName=_propertyDockPanel}"             
                 TextAlignment="Right" Foreground="Gray" TextWrapping="Wrap" Width="120" Height="20" HorizontalAlignment="Right" Margin="0 0 15 0"
                 ToolTip="{Binding Value, ElementName=_propertyDockPanel}" Template="{StaticResource _inspectorValueTemplate}"
                 LostFocus="_textBoxValue_LostFocus" GotFocus="_textBoxValue_LostFocus" KeyDown="_textBoxValue_KeyDown" TextChanged="_textBoxValue_TextChanged"         
             />
    </DockPanel>

And here is how it is used in details control:

<userControls:PropertySectionControl Property="Total" Value="{Binding OrderCharges.Total}" IsReadOnly="True"/>

As you can see, in PropertySectionControl there is also binding ToolTip to Value which is works on DataContext changed! But for TextBox.Text is not. What is this?

UPD: PropertySectionControl.cs

public partial class PropertySectionControl : DockPanel
    {
        public string Property
        {
            get { return (string)GetValue(PropertyProperty); }
            set { SetValue(PropertyProperty, value); }
        }

        public string Value
        {
            get { return (string)GetValue(ValueProperty); }
            set { SetValue(ValueProperty, value); }
        }


        public bool IsReadOnly
        {
            get { return (bool)GetValue(IsReadOnlyProperty); }
            set { SetValue(IsReadOnlyProperty, value); }
        }

        public static readonly DependencyProperty ValueProperty =
            DependencyProperty.Register("Value", typeof(string), typeof(PropertySectionControl), new PropertyMetadata(""));


        public static readonly DependencyProperty PropertyProperty =
            DependencyProperty.Register("Property", typeof(string), typeof(PropertySectionControl), new PropertyMetadata(""));


        public static readonly DependencyProperty IsReadOnlyProperty =
                    DependencyProperty.Register("IsReadOnly", typeof(bool), typeof(PropertySectionControl), new PropertyMetadata(false));


        public PropertySectionControl()
        {
            InitializeComponent();
        }


        /// <summary>
        /// Static event handler is for use in Inspector control as well
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        public static void TextBox_LostOrGotFocus(object sender, RoutedEventArgs e)
        {
            var textBox = sender as TextBox;
            if (textBox.IsFocused)
            {
                textBox.TextAlignment = TextAlignment.Left;
                textBox.SelectAll();
            }
            else
            {
                textBox.TextAlignment = TextAlignment.Right;                
            }
        }

        public static void TextBox_KeyDown(object sender, KeyEventArgs e)
        {
            var textBox = sender as TextBox;
            if (e.Key == Key.Enter)
            {
                textBox.MoveFocus(new TraversalRequest(FocusNavigationDirection.Next));
            }

            else if (e.Key == Key.Escape)
            {
                int i = 0;
                do
                {
                    textBox.MoveFocus(new TraversalRequest(FocusNavigationDirection.Right));

                } while (!(Keyboard.FocusedElement is TextBoxBase) && ++i < 5);      // prevent infinite loop

                Keyboard.ClearFocus();
            }
        }

        public static void TextBox_TextChanged(object sender, TextChangedEventArgs e)
        {
            var textBox = sender as TextBox;
            textBox.Height = 20;
            for (int i = 2; i <= textBox.LineCount; i++)
                textBox.Height += 14;
        }

        private void _textBoxValue_LostFocus(object sender, RoutedEventArgs e)
        {
            TextBox_LostOrGotFocus(sender, e);
        }

        private void _textBoxValue_KeyDown(object sender, KeyEventArgs e)
        {
            TextBox_KeyDown(sender, e);
        }

        private void _textBoxValue_TextChanged(object sender, TextChangedEventArgs e)
        {
            TextBox_TextChanged(sender, e);
        }
    }