SOAP API System.Net.WebException response stream giving BOM Characters in error response

Question

When extracting error response from System.Net.WebException using response stream. It gives BOM Characters. This happens in a few APIs and works for most APIs. The error is being fetched in Catch Section of the code. Here is my code

try
{
    using (WebResponse response = request.GetResponse())
    using (var responseStream = response.GetResponseStream())
    using (FileStream fileStream = new FileStream(filePath, FileMode.Open))
    {
        responseStream.CopyTo(fileStream);

        try // to parse the response as a valid XML
        {
            fileStream.Position = 0;

            using (XmlReader reader = XmlReader.Create(fileStream))
            {
                reader.MoveToContent();
                if (reader.LocalName == "html")
                    throw new Exception("Response from server is in html format.");
            }
        }
        catch (Exception ex)
        {
            throw new Exception("Could not fetch response from server." + ex.Message);
        }
    }
    return filePath;
}
catch (WebException ex)
{
    String responseMessage = String.Empty;
    String errorMessage = ex.InnerException != null ? ex.InnerException.Message : ex.Message;

    if (ex.Response != null)
    {
        using (var streamReader = new StreamReader(ex.Response.GetResponseStream(), Encoding.UTF8))
        {
            responseMessage = streamReader.ReadToEnd();
        }
    }
}

What could be the reason behind this?