WWDC18 "Testing Tips & Tricks" approach to testing notification center makes unit tests share state

In the following code, test 1 (test_postNotification) fails while test 2 (test_notificationsArePostedOnTheMainQueue) passes.

What concerns me, though, is that if I substitute the lines "let result = XCTWaiter.wait(for: [expectation], timeout: 0); XCTAssertEqual(result, .timedOut)" of test 2 with "wait(for: [expectation], timeout: 0.1)", then test number 1 passes.

I have cleaned the build folder and restarted Xcode and my computer, but the issue persists.

This concerns me because I would have said that the tests of the NotificationPosterTests class were isolated, but apparently they are not, since changing test 2 makes test 1 go from failing to passing.

Is this expected behavior?

import Foundation
import XCTest

extension Notification.Name {
    static let menuPostRequest = Notification.Name("menuPostRequest")
    static let editingOrderError = Notification.Name("editingOrderError")
}

class NotificationPoster {
    let notificationCenter: NotificationCenter
    
    init(notificationCenter: NotificationCenter = .default) {
        self.notificationCenter = notificationCenter
    }
    
    func postNotification(_ notification: Notification) {
        let _notificationCenter = notificationCenter // you can't use optional chaining nor conditional unwrapping on self to reference self.notificationCenter in the dispatch block because self is nil when self.postNotification(_:) is called
        DispatchQueue.main.async {
            _notificationCenter.post(notification)
        }
    }
}

final class NotificationPosterTests: XCTestCase {
    private var sut: NotificationPoster!
    private var notificationCenter: NotificationCenter!
    
    override func setUp() {
        super.setUp()
        notificationCenter = NotificationCenter()
        sut = NotificationPoster(notificationCenter: notificationCenter)
    }
    
    override func tearDown() {
        notificationCenter = nil
        sut = nil
        super.tearDown()
    }
    
    func test_postNotification() {
        let notification = Notification(name: .menuPostRequest)
        let expectation = XCTNSNotificationExpectation(
            name: notification.name,
            object: notification.object,
            notificationCenter: notificationCenter
        )
        
        sut.postNotification(notification)
        
        wait(for: [expectation], timeout: 0.1) // don't make it 0.01
    }
    
    func test_notificationsArePostedOnTheMainQueue() {
        let notification = Notification(name: .editingOrderError)
        let expectation = XCTNSNotificationExpectation(
            name: notification.name,
            object: notification.object,
            notificationCenter: notificationCenter
        )
        
        sut.postNotification(notification)
        
        let result = XCTWaiter.wait(for: [expectation], timeout: 0)
        XCTAssertEqual(result, .timedOut)
    }
}