DidEnterRegion and DidExitRegion are being called several times when monitor beacons

Question

Here is my simple code, where I'm trying to monitor enter/exit events. The problem I faced: DidEnterRegion and DidExitRegion are being called several times: the first one - when it actually happens and another calls - when I come to background and then return back to foreground in range between ~15-30 seconds after the first call of DidEnterRegion/DidExitRegion

What is the reason of this triggers? Why is this happening?

class AppDelegate: UIResponder, UIApplicationDelegate, CLLocationManagerDelegate {
    var window: UIWindow?

    var locationManager = CLLocationManager()
    var beaconRegion: CLBeaconRegion!

    func application(_ application: UIApplication, didFinishLaunchingWithOptions launchOptions: [UIApplication.LaunchOptionsKey: Any]?) -> Bool {
        confugureLocationManager()
        configureRegion()

        return true
    }

    private func confugureLocationManager() {
        locationManager.delegate = self

        if CLLocationManager.authorizationStatus() != .authorizedAlways {
            locationManager.requestAlwaysAuthorization()
        }

        print("monitored regions: \(String(describing: locationManager.monitoredRegions))")
    }

    private func configureRegion() {
        let uuid = UUID(uuidString: "04C7E2F3-42A5-5127-B066-502C8A27EB85")!
        beaconRegion = CLBeaconRegion(uuid: uuid, identifier: uuid.uuidString)
    }

    func locationManager(_ manager: CLLocationManager, didChangeAuthorization status: CLAuthorizationStatus) {
        if status == .authorizedAlways {
            if CLLocationManager.isMonitoringAvailable(for: CLBeaconRegion.self) {
                locationManager.startMonitoring(for: beaconRegion)
            }
        }
    }

    func locationManager(_ manager: CLLocationManager, didDetermineState state: CLRegionState, for region: CLRegion) {
        if state == .inside {
            print("Did determine INSIDE state for region.")
        } else {
            if state == .outside {
            print("Did determine OUTSIDE state for region.")
            }

            if state == .unknown {
                print("Did determine UNKNOWN state for region.")
            }
        }
    }

    func locationManager(_ manager: CLLocationManager, didStartMonitoringFor region: CLRegion) {
        print("did start monitoring")
    }

    func locationManager(_ manager: CLLocationManager, didEnterRegion region: CLRegion) {
        print("Did enter region: \(region.identifier), manager: \(manager)")
    }

    func locationManager(_ manager: CLLocationManager, didExitRegion region: CLRegion) {
        print("Did exit region: \(region.identifier)")
    }
}