Have you ever encountered NSFetchedResultsController not able to properly "section" based on sectionNameKeyPath, randomly?

This problem has buzzed me quite a while. So far, I still haven't founded a good solution.

Currently, I have an entity with a Bool column named pinned. I use it as the sectionNameKeyPath for NSFetchedResultsController.

So, my UICollectionView will always have 1 sections (All pinned = true, or All pinned = false), or 2 sections (Some pinned = true, and some pinned = false)

When I toggle the pinned value from true to false, or false to true, I expect FRC shall fire a "move" callback. (Due to item has moved to a new section) This happen most of the time.

However, sometimes, randomly, instead of firing "move" callback, FRC will fire "update" callback.

When such incident happen, I will exam the content of fetchedResultsController.sections. Then, I will notice the entity item stays in wrong section.

My FRC looks pretty straightforward.

Code Block 
lazy var fetchedResultsController: NSFetchedResultsController<NSPlainNote> = {
        
    // Create a fetch request for the Quake entity sorted by time.
    let fetchRequest = NSFetchRequest<NSPlainNote>(entityName: "NSPlainNote")
    fetchRequest.sortDescriptors = [
        NSSortDescriptor(key: "pinned", ascending: false)
    ]
    
    // Create a fetched results controller and set its fetch request, context, and delegate.
    let controller = NSFetchedResultsController(fetchRequest: fetchRequest,
                                                managedObjectContext: CoreDataStack.INSTANCE.persistentContainer.viewContext,
                                                sectionNameKeyPath: "pinned",
                                                cacheName: nil
    )
    controller.delegate = fetchedResultsControllerDelegate
    
    // Perform the fetch.
    do {
        try controller.performFetch()
    } catch {
        fatalError("Unresolved error \(error)")
    }
    
    return controller
}()

This is how I update the pinned column using background thread.

Code Block 
func updatePinned(_ objectID: NSManagedObjectID, _ pinned: Bool) {
    let coreDataStack = CoreDataStack.INSTANCE
    let backgroundContext = coreDataStack.backgroundContext
    backgroundContext.perform {
        let nsPlainNote = try! backgroundContext.existingObject(with: objectID) as! NSPlainNote
        
        nsPlainNote.pinned = pinned
        
        RepositoryUtils.saveContextIfPossible(backgroundContext)
    }
}

I am not really sure, whether this can caused by my background thread.

As, if I replace the backgroundContext with viewContext, I haven't observed the random problem so far.

But, even so, I am not confident to conclude using backgroundContext is the culprit to this problem.
The setup of my background thread is also pretty straightforward. I cannot see how it can went wrong.

Code Block 
class CoreDataStack {
    public static let INSTANCE = CoreDataStack()
    
    private init() {
    }
    
    lazy var persistentContainer: NSPersistentContainer = {
        let container = NSPersistentContainer(name: "***")
        container.loadPersistentStores(completionHandler: { (storeDescription, error) in
            if let error = error as NSError? {
                fatalError("Unresolved error \(error), \(error.userInfo)")
            }
        })
        
        // TODO:
        container.viewContext.automaticallyMergesChangesFromParent = true
        //container.viewContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
        //container.viewContext.undoManager = nil
        //container.viewContext.shouldDeleteInaccessibleFaults = true
        
        return container
    }()
    
    lazy var backgroundContext: NSManagedObjectContext = {
        let backgroundContext = persistentContainer.newBackgroundContext()
        
        // TODO:
        //backgroundContext.mergePolicy = NSMergeByPropertyObjectTrumpMergePolicy
        //backgroundContext.undoManager = nil
        
        return backgroundContext
    }()
}

I was wondering, have anyone of you encounter similar problem when trying to utilize sectionNameKeyPath or FRC? Do you know what? Do you have any workaround/ solution for that?

Thank you!