URLSessionDownloadDelegate didFinishDownloadingTo not called in background when download is started from action extension

I start a download in an action extension (`ActionRequestHandler`) like this:

    private lazy var urlSession: URLSession = {
        let config = URLSessionConfiguration.background(withIdentifier: "de.stefantrauth.Downloader")
        config.sharedContainerIdentifier = "group.de.stefantrauth.Downloader"
        return URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue.main)
    }()
   
    private func initiateDownloadOfFileFrom(url: URL) {
        urlSession.downloadTask(with: url).resume()
        completeRequest() // this tells the system the action extension is done with its work
    }

Then the download is processed by iOS in the background.

I now want to handle the finished download in my main application `AppDelegate`, because that is what iOS calls when the download has finished.

    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
        print("handleEventsForBackgroundURLSession")
        urlSessionBackgroundCompletionHandler = completionHandler
    }

This method gets called in background after some time as expected.

My `AppDelegate` also implements `URLSessionDelegate` and `URLSessionDownloadDelegate` to process updates for the download.

Especially interesting are

    func urlSessionDidFinishEvents(forBackgroundURLSession session: URLSession) {
        DispatchQueue.main.async {
            print("urlSessionDidFinishEvents")
            self.urlSessionBackgroundCompletionHandler?()
            self.urlSessionBackgroundCompletionHandler = nil
        }
    }

and

    func urlSession(_ session: URLSession, downloadTask: URLSessionDownloadTask, didFinishDownloadingTo location: URL) {
        print("download finished to \(location.absoluteString)")
        do {
            let documentsURL = try FileManager.default.url(for: .documentDirectory, in: .userDomainMask, appropriateFor: nil, create: false)
            let savedURL = documentsURL.appendingPathComponent(location.lastPathComponent)
            try FileManager.default.moveItem(at: location, to: savedURL)
            print("moved file to: \(savedURL.absoluteString)")
        } catch {
            print ("file error: \(error)")
        }
    }

Both `urlSessionDidFinishEvents` and `didFinishDownloadingTo` are not being called after `handleEventsForBackgroundURLSession` got called in background. Only after relaunching the app into foreground the delegate methods get called.

Why are they not getting called and what can I do to fix that?

I tried creating the `URLSession` in `handleEventsForBackgroundURLSession`like this:

    private func initUrlSessionWith(identifier: String) {
        let config = URLSessionConfiguration.background(withIdentifier: identifier)
        config.sharedContainerIdentifier = "group.de.stefantrauth.Downloader"
        urlSession = URLSession(configuration: config, delegate: self, delegateQueue: OperationQueue.main)
    }


    func application(_ application: UIApplication, handleEventsForBackgroundURLSession identifier: String, completionHandler: @escaping () -> Void) {
        print("handleEventsForBackgroundURLSession")
        initUrlSessionWith(identifier: identifier)
        urlSessionBackgroundCompletionHandler = completionHandler
    }

However this did not fix the problem.

Before you ask: Yes I am testing this on a real device because the simulator has problems with background task handling.