ios – Why does cancelByProducingResumeData() return nil for upload tasks?

I’m implementing resumable uploads on iOS based on this documentation and wwdc video

The server correctly sends a 104 Upload Resumption Supported informational response, which I receive in my delegate. However, when I cancel the upload task using cancelByProducingResumeData(), it always returns nil.

Setup

• iOS 18/26
• HTTP/2 server with TLS (using SwiftNIO + Network.framework)
• Server implements the resumable upload draft protocol

Client Code

class VideoUploader: NSObject, URLSessionTaskDelegate, URLSessionDataDelegate {
    private var urlSession: URLSession!
    private var uploadTask: URLSessionUploadTask?

    func uploadVideo(fileURL: URL) {
        var request = URLRequest(url: URL(string: "
        request.httpMethod = "POST"
        request.setValue("?0", forHTTPHeaderField: "Upload-Incomplete")
        request.setValue("3", forHTTPHeaderField: "Upload-Draft-Interop-Version")

        uploadTask = urlSession.uploadTask(with: request, fromFile: fileURL)
        uploadTask?.resume()

        // Cancel after 3 seconds to test resume data
        Task {
            try? await Task.sleep(for: .seconds(3))
            guard let uploadTask else { return }
            let resumeData = await uploadTask.cancelByProducingResumeData()
            print("Resume data: \(resumeData)") // Always prints nil
        }
    }

    // This IS called - server sends 104 correctly
    func urlSession(_ session: URLSession, task: URLSessionTask, didReceiveInformationalResponse response: HTTPURLResponse) {
        print("Received 104: \(response.statusCode)")
        print("Location: \(response.allHeaderFields["Location"] ?? "none")")
    }
}

Here’s a repo with server side code and app that reproduces the issue.

Any idea what I could be missing, or even how I could debug this?