Google App Engine, Spring and session bean

I created very simple Spring application with one session bean. Sources for : bean class (in Google App Engine environment it requires to be Serializable), servlet source code and web.xml (whole project source code) . I expected that every page refresh will increase the id attribute in PersonBean. But it works only in development environment, after deploying to production server the bean is persisted only for the first time (it keep number 1) - any next refresh does not change it, as is the first bean version has been persisted for good.
It was changed after setting additional attribute to session which does have not any meaning.

req.getSession().setAttribute("attrib", "X");

After adding this setting it works as expected also in production environment. I can hardly understand this behaviour - it looks that setting any attribute to session triggers also session bean being persisted at the end of the request.