Datepicker, serious problems

Hi all together,
i have serious Problems with Datepicker.

ProjectWorkTimeDetailViewModel.cs
public class ProjectWorkTimeDetailViewModel
{
more code..................
public override void OnNavigatedTo(NavigationParameters parameters)
{
base.OnNavigatedTo(parameters);
if (parameters.ContainsKey("PWT"))
{
var projectWorkTime = (ProjectWorkTime)parameters["PWT"];
CurrentWorkTime = new ProjectWorkTimeEdit(projectWorkTime);
CurrentWorkTime.PropertyChanged += OnPropertyChange;
}
}

private void OnPropertyChange(object sender, PropertyChangedEventArgs e)
    {
        if (!Title.EndsWith("*"))
        {
            Title = Title + " *";
        }
        _canSave = true;
        SaveCommand.RaiseCanExecuteChanged();
    }

}

ProjectWorkTimeDetail.xaml

                <ViewCell>
                    <StackLayout Orientation="Horizontal"   Padding="13,0">
                        <Label Text="Arbeitstag:  " VerticalOptions="Center"></Label>

                        <DatePicker Format="dd MMM yyyy"
                                Date="{Binding CurrentWorkTime.WorkDay,Mode=TwoWay}"
                                HorizontalOptions="FillAndExpand"></DatePicker>
                    </StackLayout>
                </ViewCell>

ProjectWorkTimeEdit.cs
public class ProjectWorkTimeEdit : BindableBase
{
ProjectWorkTime _entity;

    public ProjectWorkTimeEdit (ProjectWorkTime entity)
    {
        if (entity == null)
        {
            throw new ArgumentNullException(nameof(entity));
        }

        _entity = entity;
       _workDay = entity.WorkDay;
        _job = _entity.Job;
        _workTime = _entity.WorkTime;
    }

    DateTime _workDay;
    public DateTime WorkDay
    {
        get => _workDay;
        set => SetProperty(ref _workDay,value);
    }

........... more code
}

As you can see i'm initializing 'ProjectWorkTimeEdit.cs' and set values in the Constructor. After that, i add the 'OnPropertyChanged' EventHandler.

Directly after this Line:
CurrentWorkTime.PropertyChanged += OnPropertyChange;

I have a Event fired from

 public DateTime WorkDay
{
     set => SetProperty(ref _workDay,value);
}

here without doing anything

I'm shure that this problem has something todo with the Datepicker because if i commented out the Datepicker all works as expected.

Any idea???

Thanks in advance
Peter