C# WPF Combobox: DisplayMemberPath is not updated after change of SelectedValue

Question

i have an issue with a Combobox: After a property of the class object corresponding with the XAML property "SelectedValue" has been changed, the user-visible / displayed text (specified by "DisplayMemberPath") is not updated - the user stays to see the old value.

Here is the XAML part:

<ComboBox x:Name="PM_CB_PackageVersion"
ItemsSource="{Binding Source={StaticResource MainWindowPackageViewModel}, Path=PM_CB_PackageVersion}"
SelectedValue="{Binding Source={StaticResource MainWindowPackageViewModel}, Path=Selected_PM_CB_PackageVersion, UpdateSourceTrigger=PropertyChanged}"
IsEnabled="{Binding Source={StaticResource MainWindowPackageViewModel}, Path=IsEnabled_PM_CB_PackageVersion}"
DisplayMemberPath="AppPackageVersion"
HorizontalAlignment="Left" VerticalAlignment="Top" Margin="125,147,0,0" Width="130"/>

I debugged, and the strange thing is: I defined an "OnPropertyChanged" event, which is correctly reached after i changed the value of "Selected_PM_CB_PackageVersion.AppPackageVersion", and i can see while debugging that the "AppPackageVersion" is set to the new value - everything looks fine. But in the WPF Form the user continues to see the old value.

How can i make sure that what the user sees - controlled by "DisplayMemberPath" - is refreshed after i changed the corresponding Property in the background?

Thanks + Best regards !