IDFA iOS14 returns denied without displaying auth popup

I am doing a research on getting the IDFA on iOS 14. I am using iPhone 8 Plus.

I have added

<key>NSUserTrackingUsageDescription</key>
<string>App would like to access IDFA for tracking purpose</string>

in the .plist file.

Then added

let type = ATTrackingManager.trackingAuthorizationStatus;

which returns .denied, having

func requestPermission() {
        ATTrackingManager.requestTrackingAuthorization { status in
            switch status {
            case .authorized:
                // Tracking authorization dialog was shown
                // and we are authorized
                print("Authorized")
            
                // Now that we are authorized we can get the IDFA
            print(ASIdentifierManager.shared().advertisingIdentifier)
            case .denied:
               // Tracking authorization dialog was
               // shown and permission is denied
                 print("Denied")
            case .notDetermined:
                    // Tracking authorization dialog has not been shown
                    print("Not Determined")
            case .restricted:
                    print("Restricted")
            @unknown default:
                    print("Unknown")
            }
        }
    }

But I'm getting .denied without any popup.

Do you know what is happening?

There's an option "Allow Apps to Request to Track" in system's Settings app, and if it's off, requestTrackingAuthorization will return .denied immediately.