Using Flurl's OnError to check for custom response headers in ASP.NET Core Web API

I am developing a sort of middleware API, which is meant to connect to various systems. I am using the Flurl library to make requests to the individual systems.

One of these systems sometimes returns a custom header containing an error message, rather than just returning it in the response body (for reasons unbeknownst to me).

To return the correct error message, I am using Flurl's OnError event handler. I defined behaviour which checks the response for this custom header and, if found, extracts the decoded message and sets it as the response message of the request:

/// <summary>
/// Asynchronously checks responses from Thinkwise Indicium for TSFMessage headers and, if found,
/// decodes them and sets them as the actual error message of the response.
/// </summary>
/// <param name="request">The request of which the response must be checked for a Thinkwise message header.</param>
/// <returns>A Task.</returns>
public static async Task HandleThinkwiseErrorAsync(FlurlCall request)
{
  if (request.Response.Headers.TryGetFirst("TSFMessages", out var message))
  {
    var decodedMessage = JsonSerializer.Deserialize<ThinkwiseSoftwareFactoryMessage>(
                       Encoding.UTF8.GetString(Convert.FromBase64String(message)));

    var response = new HttpResponseMessage
    {
      StatusCode = HttpStatusCode.BadRequest,
      RequestMessage = new HttpRequestMessage(request.Request.Verb, request.Request.Url),
      Content = new StringContent(
        decodedMessage?.RawMessage.Split("\"")[1]
        ?? await request.Response.ResponseMessage.Content.ReadAsStringAsync(default))
      };

    request.HttpResponseMessage = response;
    request.ExceptionHandled = false;
  }
}

While I do have prior experience making HTTP requests using Flurl, configuring it like this is a first for me. I have configured Flurl as such in the DI container:

FlurlHttp.Configure(settings =>
{
  settings.OnErrorAsync = IndiciumFlurlHelper.HandleThinkwiseErrorAsync;
  settings.OnError = IndiciumFlurlHelper.HandleThinkwiseError; // Same as async method
});

I am having difficulty finding other articles, issues or SO questions which are trying to do similar things. I am probably making a rookie mistake somewhere, or just misunderstanding how the FlurlCall object is constructed and what the properties mean.

The problem I am running into is that, when I make a request I know should return an error message using this header, my middleware API returns a 204 response when it should return a 400 response with the error message in the body.

What am I doing wrong? The requests seem to work as expected, so I am pretty sure I am doing something wrong handling this response header this way.

Thanks in advance! Please do leave a comment, should you have further questions.