Callback of location permission popup not get called

I have cerated a wrapper class of CLLocationManager

public class MyLocationManager: NSObject, CLLocationManagerDelegate {

  public static let shared = MyLocationManager()

  var locationManager: CLLocationManager

  override init() {
     locationManager = CLLocationManager()
     super.init()
     locationManager.delegate = self
  }

  // function to request location “when-in-use” permission 
  public func requestWhenInUse() {
        locationManager.requestWhenInUseAuthorization()
    }

  // callback of location permission popup
  private func locationManager(_ manager: CLLocationManager, didChangeAuthorizationStatus status: CLAuthorizationStatus) {
        //PROBLEM: it never get called
        print(“Status: \(status)")
    }
}

In one of my UIViewController, I request location permission by:

override func viewDidAppear(_ animated: Bool) {
    MyLocationManager.shared.requestWhenInUse()
}

When run app, location permission popup comes, but after allow or deny permission, the callback inside MyLocationManager never get called, why?

I take this as a reference: https://developer.apple.com/documentation/corelocation/cllocationmanagerdelegate/1423701-locationmanager