iOS NSFetchedResultsController updating incorrectly with background context

Question

I have a table view hooked up to a NSFetchedResultsController:

self.fetchedResultsController = [[NSFetchedResultsController alloc] initWithFetchRequest:request
                                                                        managedObjectContext:self.database.managedObjectContext
                                                                      sectionNameKeyPath:nil
                                                                               cacheName:nil];

I create entities in a background context like this:

    NSManagedObjectContext *backgroundContext;
    backgroundContext = [[NSManagedObjectContext alloc] initWithConcurrencyType:NSPrivateQueueConcurrencyType];
    backgroundContext.parentContext = document.managedObjectContext; 

    [backgroundContext performBlock:^{
        [MyAPI createEntitiesInContext:backgroundContext];

        NSError *error = nil;
        [backgroundContext save:&error];
        if (error) NSLog(@"error: %@",error.localizedDescription);

        [document.managedObjectContext performBlock:^{
            [document updateChangeCount:UIDocumentChangeDone];
            [document.managedObjectContext save:nil];
        }];

Now, whenever I get new data (and insert/update entities like shown just above), my NSFetchedResultsController doesn't quite work as it should. In particular, I am always updating one entity (not creating a new one), but my table view shows two entities. Once I restart the app it shows up correctly.

If I perform the creation of the entities ([MyAPI createEntities]) in self.database.managedObjectContext, everything works fine.

What am I doing wrong? I've been stuck with this for hours now.