Progress closure seems broken

[LinusGeffarth]

I'm using the following code to upload an image in base64 format and I would like to track that upload's progress:

var r  = URLRequest(url: URL(string: apiURL+"images")!)
r.httpMethod = "POST"
r.setValue("multipart/form-data", forHTTPHeaderField: "Content-Type")
r.setValue(accessToken, forHTTPHeaderField: "Authorization")
r.httpBody = {
    var body = Data()
    let boundary = "Boundary-\(UUID().uuidString)"
    let boundaryPrefix = "--\(boundary)\r\n"
    
    for (key, value) in ["file": imageData] {
        body.append(string: boundaryPrefix)
        body.append(string: "Content-Disposition: form-data; name=\"\(key)\"\r\n\r\n")
        body.append(string: "\(value)\r\n")
    }
    body.append(string: "\r\n")
    body.append(string: "--".appending(boundary.appending("--")))
    
    return body
}()
let task = HTTP(r)
task.progress = { progress in
    print(progress)
}
task.run { (response) in
    print(response.text)
}

What this prints:

1.0
-30.0
"{some json}"

Do you know why that is? The image's size is 1.5mB, and my upload speed is around 300kB/s so it can't be at 1 immediately, not to speak about the progress of -30...

Thanks!