FCM Token automatically changed after update location while receiving silent push notification

I create an application in flutter for update the current location of user while device receive a silent push notification. Its working fine in Android and IOS. But in iOS (only app killed/terimtaed stage) after updating the location when the silent push notification receive, the device fcm token automatically changed. Why this happened? I think its security issue in IOS. The code I written in AppDelegate is given below.

Code Block 
 public override func application(_ application: UIApplication, didReceiveRemoteNotification userInfo: [AnyHashable: Any],
                     fetchCompletionHandler completionHandler: @escaping (UIBackgroundFetchResult) -> Void) {
    if let myData = userInfo["ids"] as? NSString {
        fcmUSerId = myData
        
            }
  
    locationManager?.allowsBackgroundLocationUpdates = true
 
    Messaging.messaging().delegate = self
    setupLocationManager()
    locationManager.delegate = self
      completionHandler(UIBackgroundFetchResult.newData)
       
    }
func setupLocationManager(){
                
                locationManager = CLLocationManager()
                locationManager.allowsBackgroundLocationUpdates = true
                self.locationManager?.requestAlwaysAuthorization()
                locationManager?.desiredAccuracy = kCLLocationAccuracyNearestTenMeters
                locationManager?.startUpdatingLocation()
                locationManager.pausesLocationUpdatesAutomatically = false
            }
    func locationManager(_ manager: CLLocationManager, didUpdateLocations locations: [CLLocation]) {
    
            print("location start background")
     
                   let locationValue:CLLocationCoordinate2D = manager.location!.coordinate
    
                   print("locations = \(locationValue)")
            
            let date : Date = Date()
            let dateFormatter = DateFormatter()
            dateFormatter.dateFormat = "dd-MM-yyyy hh:mm a"
            let todaysDate = dateFormatter.string(from: date)
     
            let db=Firestore.firestore()
             
            Messaging.messaging().delegate = self
    
                let token = Messaging.messaging().fcmToken
                print("FCM token: \(token ?? "")")
          db.collection("users").document(fcmUSerId! as String).setData(["startTime": todaysDate,"lastStatus": "Updated","latitude":locationValue.latitude,"longitude":locationValue.longitude,"deviceid":token],merge: true)
            
          
                   locationManager?.stopUpdatingLocation()
           }