Completion Port thread leaks when client close the connection?

var _config = new HttpSelfHostConfiguration(ConfigurationManager.AppSettings["HttpApi.ListeningURL"])
{
    TransferMode = TransferMode.StreamedResponse
};

I have a WebApi self-host application is configured to streaming the response.

And there is a HTTP GET action returns PushStreamContent

[HttpGet]
public HttpResponseMessage GET(string domains, int version = 1)
{   
    // ...
    Func<Stream, HttpContent, TransportContext, Task> func = (stream, httpContent, transportContext) =>
    {
        return Monitor(domainIDs, itemsPerDomain, version, httpContent, stream);
    };


    HttpResponseMessage response = Request.CreateResponse();

    response.StatusCode = HttpStatusCode.OK;
    response.Content = new PushStreamContent(func, new MediaTypeHeaderValue(CONTENT_TYPE));
    return response;
}

And the response is sent asynchronously

async Task Monitor(List<long> domainIDs
    , Dictionary<long, List<T2>> itemsPerDomain
    , int version
    , HttpContent httpContent
    , Stream stream
    )
{
    try
    {
        using (StreamWriter sw = new StreamWriter(stream, new UTF8Encoding(false)))
        {            
            sw.WriteLine("xxx");
            // ...
        }
    }
    catch (CommunicationException)
    {
        // ignore
    }
    catch (Exception ex)
    {
        log.Error(ex);
    }
    finally
    {
        stream.Close();
        stream.Dispose();
        httpContent.Dispose();
        log.Info("Dispose");
    }
}

If client terminates the connection when the server is sending the response.  

An CommunicationException exception is thrown and its internal exception is System.Net.HttpListenerException. error code is1229.

You can note that I dispose everything possible in finally block.

but it still leaks completion port threads.

I monitor the completion port threads via

int availableCompletionPortThreads;
ThreadPool.GetAvailableThreads(out unused, out availableCompletionPortThreads)

The availableCompletionPortThreads does not restore when the above exception occurs no matter how long I wait. And it only happens for this kind of exception -- client terminates the connection.  And it can bring down the application by exhausting the  completion port threads.

Why is the completion port thread is leaking?