strange behavior IsbussyIndicator

namespace SimpleSteps {
  public class HideDataFormCommandVisibilityConverter : IValueConverter {
 
    public HideDataFormCommandVisibilityConverter() {
      CommandButtonsVisibility = DataFormCommandButtonsVisibility.All;
    }
 
    public DataFormCommandButtonsVisibility? CommandButtonsVisibility {get; set;} 
 
    public object Convert(object value, System.Type targetType, 
         object parameter, System.Globalization.CultureInfo culture) {
      if (value.GetType() != typeof(bool)) return null;
      return ((bool) value) ? DataFormCommandButtonsVisibility.None : CommandButtonsVisibility;
    }
 
    public object ConvertBack(object value, System.Type targetType, 
         object parameter, System.Globalization.CultureInfo culture) {
      return null;
    }
 
  }
}

<UserControl  x:Class="SimpleSteps.MainPage"
    ...
    xmlns:local="clr-namespace:SimpleSteps" >




    <UserControl.Resources>
        <local:HideDataFormCommandVisibilityConverter 
            x:Key="HideEmployeeCommandsConverter" 
            CommandButtonsVisibility="Navigation, Add, Delete, Cancel, Commit"/>
    </UserControl.Resources>

3) Bind DataForm's CommandButtonsVisibility

<toolkit:DataForm Name="EmployeeDataForm" Grid.Row="0" 
                  ItemsSource="{Binding Employees}" 
                  EditEnded="dataForm1_EditEnded"
                  CurrentItem ="{Binding CurrentEmployee, Mode=TwoWay}" 
                  CommandButtonsVisibility="{Binding IsBusy, 
                    Converter={StaticResource HideEmployeeCommandsConverter}, Mode=TwoWay}"
                  />