WPF MVVM ComboBox SelectedItem or SelectedValue not working
Have you tried implementing INotifyPropertyChanged in your viewmodel, and then raise the PropertyChanged event when the SelectedItem gets set?
If this in itself doesn't fix it, then you will be able to manually raise the PropertyChanged event yourself when navigating back to the page, and that should be enough to get WPF to redraw itself and show the correct selected item.