bluetooth background mode IOS when the screen is locked

Question

I would like to implement background bluetooth scanning on IOS. When the application goes in background mode it calls TestCentralManagerDelegate which implements DiscoveredPeripheral function. It is triggered when a new bluetooth peripheral device is detected. If a new bluetooth device is detected the application read the manufacture data which is presented in Dictionary advertisementData (as argument of DiscoveredPeripheral function). The manufacture data are obtained by calling ManufactureData = advertisementData["kCBAdvDataManufacturerData"].ToString(). The discovering of the manufacture data was tested on two different iPhones 5s and 6 with the same iOS 12.1. When the application goes in background mode, I locked the screen. In the case of iPhone 5s, I observed that ManufactureData was found each time the DiscoveredPeripheral function is triggered. This fact is not true for iPhone 6, each time I got ManufactureData = null. It is worth mentioning that the manufacture data are received in both cases if the screen is not locked.

I do not understand why the iPhone 6 does not find ManufactureData, meanwhile the iPhone 5s does. I would accept the fact that phones have different operating systems and this implies different responses, but in my case this is not the case. I will appreciate any help for better understanding aforementioned problem.

Here is code I am using Xamarin.iOS.

public override  void DiscoveredPeripheral(CBCentralManager central, CBPeripheral peripheral, NSDictionary advertisementData, NSNumber RSSI)
        {

            try
            {
                central.StopScan();

                if (peripheral == null || advertisementData == null)
                {
                    central.ScanForPeripherals(cbuuids);
                    return;
                }

                string ManufactureData;

                if (advertisementData.ContainsKey(new NSString("kCBAdvDataManufacturerData")))
                {
                    ManufactureData = advertisementData["kCBAdvDataManufacturerData"].ToString();
                }
                else
                {

                    ManufactureData = null;

                    CrossLocalNotifications.Current.Show("no advertising data", "no advertising data", 10);
                    central.ScanForPeripherals(cbuuids);
                    return;
                }


                central.ScanForPeripherals(cbuuids);
            }
            catch
            {
                central.ScanForPeripherals(cbuuids);
            }

        }