iOS 10 UNNotification doesn't show alert

The code is very easy:

func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplicationLaunchOptionsKey: Any]?) -> Bool {
    UNUserNotificationCenter.current().requestAuthorization(options: [.alert, .sound, .badge], completionHandler: { (granted, error) in
        if !granted {
            print("Not allowed")
        }
    })

    let content = UNMutableNotificationContent()
    content.title = "Alert"
    content.sound = UNNotificationSound.default()

    let trigger = UNTimeIntervalNotificationTrigger(timeInterval: 20, repeats: false)

    let request = UNNotificationRequest(identifier: "test", content: content, trigger: trigger)

    UNUserNotificationCenter.current().add(request, withCompletionHandler: nil)
    return true
}

It works well on iOS 11, like this:

but on iOS 10, the alert doesn't show.

On both iOS 10 and iOS 11, the sound did appear.

My Xcode version is 9.2(9C40b)

Any help is appreciated.

Try to add the body of notification like this

content.body = "Any text/Blank Space"

Hope this will help you