Registering to receive notifications of remote CloudKit changes not working

You observed the persistent container instead of the store coordinator, it should be this:

[NSNotificationCenter.defaultCenter addObserver:self selector:@selector(persistentStoreRemoteChangeNotification:) name:NSPersistentStoreRemoteChangeNotification object:_persistentContainer.persistentStoreCoordinator];

Wherever you access your app’s persistent CloudKit container to grab the viewContext, you need to set the automaticallyMergesChangesFromParent property to true.

lazy var managedContext: NSManagedObjectContext = {
    self.storeContainer.viewContext.automaticallyMergesChangesFromParent = true
    return self.storeContainer.viewContext

Making this one-line change will enable the app (which is supported by NSFetchedResultsController) to update the UI in response to remote data changes…