NSDiffableDataSourceSnapshot bug?

i found something really odd in NSDiffableDataSourceSnapshot behaviour. the tl;dr version is that this:

                snapshot.moveItem("one", afterItem: "two")
                dataSource.apply(snapshot)

gives correct result, while this:

                var snapshotCopy = snapshot
                snapshotCopy.moveItem("one", afterItem: "two")
                dataSource.apply(snapshotCopy)

gives incorrect result, despite being semantically equivalent. snapshot is a true value type, right?

it is as odd as if i wrote:

var b = a
b += 1
a = b

and got a different result in "a" compared to "a += 1"

minimal source fragment attached (doesn't need nib files/storyboards)
========

Code Block 
import UIKit
class ViewController: UIViewController {
		var tv: UITableView!
		var dataSource: UITableViewDiffableDataSource<String, String>!
		override func viewDidLoad() {
				super.viewDidLoad()
				tv = UITableView(frame: view.bounds)
				tv.autoresizingMask = [.flexibleWidth, .flexibleHeight]
				view.addSubview(tv)
				tv.register(UITableViewCell.self, forCellReuseIdentifier: "cell")
		
				var originalSnapshot = NSDiffableDataSourceSnapshot<String, String>()
				originalSnapshot.appendSections(["***"])
				originalSnapshot.appendItems(["one", "two", "three"])
		
				dataSource = UITableViewDiffableDataSource<String, String>(tableView: tv) { (tv, indexPath, item) -> UITableViewCell? in
						let cell = tv.dequeueReusableCell(withIdentifier: "cell", for: indexPath)
						cell.textLabel?.text = item
						return cell
				}
		
				dataSource.apply(originalSnapshot)
		
				DispatchQueue.main.asyncAfter(deadline: .now() + 2) {
						var snapshot = self.dataSource.snapshot()
						snapshot.insertItems(["four"], afterItem: "one")
				
						if true {
								var snapshotCopy = snapshot
								snapshotCopy.moveItem("one", afterItem: "two")
								self.dataSource.apply(snapshotCopy)
								/* gives incorrect result:
								two, one, three */
						}
						else {
								snapshot.moveItem("one", afterItem: "two")
								self.dataSource.apply(snapshot)
								/* gives correct result: 
								four, two, one, three */
						}
				}
		}
}
@UIApplicationMain
class AppDelegate: UIResponder, UIApplicationDelegate {
		
		var window: UIWindow?
		
		func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
				
				window = UIWindow(frame: UIScreen.main.bounds)
				window!.rootViewController = ViewController()
				window!.makeKeyAndVisible()
				return true
		}
}