Webclient headers not adding in iOS build

Hello all. I have to add headers to a webclient async request and everything seems to work in the editor, but whenever I create a build for a device (iOS) the header don't seem to "stick" for the native requests. For example I do this:

    private static readonly string headerKey = "123456789";
 
    public static WebClient client;
 
     public static void DownloadString()
     {
                       using(client = new WebClient())
             {
                 client.Headers.Add("Authorization", headerKey);
                 client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(DownloadStringCallback);
                 client.DownloadStringAsync(new Uri(nextURL));
             }
     }
 
     public static void DownloadStringCallback(object sender, DownloadStringCompletedEventArgs args)
     {
               //Do stuff
     }

And this all works fine in the editor. I can get a response to the DownloadStringCallback function and the args parameter contains a valid args.Result

I know this is a problem with the headers being added because I get a response on device but it is a null result. The headers act as a key for a reverse proxy that we have set-up, and when I hit our endpoint directly with a device build I get valid responses just like in the editor.

Is this just a problem with Unity's translation of a WebClient request into a native iOS request?

If so can I accomplish the same async functionality with another request class? I'm thinking of HttpWebRequest but I don't know if I am going to have the same problems.