WPF DataGridComboBoxColumn 1失去焦点就丢失text

WPF DataGridComboBoxColumn 一失去焦点就丢失text
在我的DataGrid中定义两个级联的DataGridComboBoxColumn，A1，A2，A1变的时候也把A2的ItemsSource改变。现在改变起来没问题，但是A2一旦失去焦点，那么本应显示的Text也不显示，十分奇怪。背后的绑定显示数据更改已经生效了，但是就是不显示在DataGrid上：

<DataGrid x:Name="Dg_Main" HorizontalScrollBarVisibility="Auto" VerticalScrollBarVisibility="Auto"

                                CanUserAddRows="True" CanUserDeleteRows="True" SelectionMode="Single" SelectionUnit="FullRow"

                                AlternatingRowBackground="Wheat" RowHeaderWidth="10" AlternationCount="2"

                                AutoGenerateColumns="False" ItemsSource="{Binding Path=Displays}" PreparingCellForEdit="Dg_Main_PreparingCellForEdit">

                            <DataGrid.Columns>

                                <DataGridComboBoxColumn Header="A1" MinWidth="80" ItemsSource="{Binding Source={StaticResource A1Source}}" SelectedValueBinding="{Binding Path=a1, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" SelectedValuePath="PrimitiveValue" DisplayMemberPath="Content"/>

                                <DataGridComboBoxColumn Header="A2" MinWidth="80"   SelectedValueBinding="{Binding Path=a2, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" SelectedValuePath="PrimitiveValue" DisplayMemberPath="Content" EditingElementStyle="{StaticResource EditCombobox}">

                                    <DataGridComboBoxColumn.ElementStyle>

                                        <Style TargetType="ComboBox">

                                            <!--<Setter Property="ItemTemplate">

                                                <Setter.Value>

                                                    <DataTemplate>

                                                        <TextBlock Text="{Binding Path=Content}"></TextBlock>

                                                    </DataTemplate>

                                                </Setter.Value>

                                            </Setter>-->

                                            <!--<Setter Property="DisplayMemberPath" Value="{Binding Path=Content}"></Setter>-->

                                            <Setter Property="IsSynchronizedWithCurrentItem" Value="False" />