Check out as draft does't work properly

Hi experts,

I have to create a custom Response.Filter for SharePoint server.

There is an issue when I do a checkout as draft and a checkin as following image

My environment:

Server: windows 2003, sharepoint 2007

Client: windows xp sp3 and MS Office 2007

I also attached my httpmodule as: 

public class HelloFilter : IHttpModule
    {
        public void Init(HttpApplication application)
        {
            application.PreRequestHandlerExecute += new EventHandler((this.Application_PreRequestHandlerExecute));
        }


        string ResponseFilter(string output)
        {
            return output;
        }


        private void Application_PreRequestHandlerExecute(Object source, EventArgs e)
        {
            HttpApplication application = (HttpApplication)source;
            HttpContext context = application.Context;
            HttpRequest req = context.Request;


            if (req.RawUrl.ToLower().Contains("author.dll") && req.RequestType.ToLower() == "post" &&
                    req.UserAgent == "MSFrontPage/12.0")
            {
                ResponseFilterStream filter = new ResponseFilterStream(application.Response.Filter);
                filter.TransformString += ResponseFilter;
                application.Response.Filter = filter;
            }
        }


        public void Dispose() { }


    }

Every reply would be appreciated!

Ngo Duc Huy