The current realization of AppSettingsStorageMechanism.Delete method deletes entry by the key. Probably body of this method should be empty. The isolated storage entry should live forever.
For instance you have a SecondViewModel that could be opened from MainViewModel and corresponding SecondViewModelStorage : StorageHandler<SecondViewModel>. In Configure method you specify that Comment property should be stored in application settings:
public override void Configure()

    {
        Property(aModel => aModel.Comment).InAppSettings();
    }

First scenario:

1. Navigate to SecondPageView from MainPageView and set some value to Comment field.
2. Navigate back to MainPageView and then navigate out of application (press back button again).
3. Open application one more time and navigate ro SecondPageView.
4. Notice Comment field has value.
   

Second scenario:

1. Navigate to SecondPageView from MainPageView and set some value to Comment field.
2. Navigate back to MainPageView.
3. Navigate to SecondPageView again.
4. Notice Comment field is blank.
   

If a value survive application restart it should survive navigation during same application session.