I use method ”dropdownopen“ to refresh combobox，but the selectedvalue disappear，how to solve it?

1.I want to refresh the comboboxitem when i dropdown the combobox, so i add eventtrigger to excute dropdownopen command,here is the code of xaml,vm,model (1)xaml enter image description here

<DataGrid ItemsSource="{Binding BSFinalWorkday}" AutoGenerateColumns="False" CanUserAddRows="False">
                            <DataGrid.Columns>
                                <DataGridTextColumn Header="Shop" Binding="{Binding WorkshopID}" IsReadOnly="True"/>
                                <!--M1--> 
                                <DataGridTemplateColumn Header="M1">
                                    <DataGridTemplateColumn.CellTemplate>
                                        <DataTemplate>
                                            <ComboBox x:Name="m1combox" SelectedValue="{Binding Jan,UpdateSourceTrigger=PropertyChanged}"
                                                      ItemsSource="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGrid}}, Path=DataContext.SingleBSShift}"
                                                      SelectedValuePath="TotalWorkday" DisplayMemberPath="TotalWorkday" >
                                                <i:Interaction.Triggers>
                                                    <i:EventTrigger EventName="DropDownOpened">
                                                        <i:InvokeCommandAction  
                                                            Command="{Binding RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type DataGrid}}, Path=DataContext.InitSingleBSShiftCmdString}" >
                                                            <i:InvokeCommandAction.CommandParameter>
                                                                <MultiBinding Converter="{ StaticResource ResourceKey=objectConverter}" Mode="TwoWay">
                                                                    <MultiBinding.Bindings>
                                                                        <Binding Path="WorkshopID"/>
                                                                        <Binding Path="M1"/>
                                                                    </MultiBinding.Bindings>
                                                                </MultiBinding>
                                                            </i:InvokeCommandAction.CommandParameter>
                                                        </i:InvokeCommandAction>
                                                    </i:EventTrigger>
                                                    
                                                </i:Interaction.Triggers>
                                            </ComboBox>
                                        </DataTemplate>
                                    </DataGridTemplateColumn.CellTemplate>
                                </DataGridTemplateColumn>

(2)viewmodel`private ObservableCollection singleBSShift;

    public ObservableCollection<BSAvailableShiftWorkday> SingleBSShift 
    {
        get { return singleBSShift; }
        set { singleBSShift = value; RaisePropertyChanged(); }
    }

    public DelegateCommand<object> InitSingleBSShiftCmdString { get; set; }
    public void InitSingleBSShiftMethodString(object obj)
    {
        object[] multiObj = obj as object[];
        string wID = multiObj[0] as String;
        string month = multiObj[1] as String;
        Console.WriteLine("Hi");
        Console.WriteLine(wID);
        Console.WriteLine(month);
        if (BSAvailableShift != null)
        {
            SingleBSShift.Clear();
            var res = BSAvailableShift.Where(p => p.WorkshopID == wID && p.Month== month).Select(p => p);
            foreach (var b in res) { SingleBSShift.Add(b); }
        }
    }`

（3）model

public class BSAvailableShiftWorkday : BindableBase { private string workshopID;

    public string WorkshopID
    {
        get { return workshopID; }
        set { workshopID = value; RaisePropertyChanged(); }
    }

    private string  month;

    public string  Month
    {
        get { return month; }
        set { month = value; RaisePropertyChanged(); }
    }

    private string totalWorkday;

    public string TotalWorkday
    {
        get { return totalWorkday; }
        set { totalWorkday = value; RaisePropertyChanged(); }
    }

2.When i selected row's combobox, it truly shows the selectedvalue,but when i click next row's combobox, the above selectedvalue disappear enter image description here