WPF MVVM ComboBox SelectedItem or SelectedValue not working

Question

Update

After a bit of investigating. What seems to be the issue is that the SelectedValue/SelectedItem is occurring before the Item source is finis

Answer 1

When leaving the current page, the CollectionView associated with the ItemsSource property of the ComboBox is purged. And because the ComboBox IsSyncronizedWithCurrent property is true by default, the SelectedItem and SelectedValue properties are reset.
This seems to be an internal data type issue in the binding. As others suggested above, if you use SelectedValue instead by binding to an int property on the viewmodel, it will work. A shortcut for you would be to override the Equals operator on MyObject so that when comparing two MyObjects, the actual Id properties are compared.

Another hint: If you do restructure your viewmodels and use SelectedValue, use it only when SelectedValuePath=Id where Id is int. If using a string key, bind to the Text property of the ComboBox instead of SelectedValue.