What you're experiencing is the expected behavior in a portlet environment.
In a portlet the Portlet Bridge makes JSF behave very differently to support portlet lifecycle calls. In a nutshell, when you change the value from false to true on the page of the portlet that has occurred in an ActionRequest. The Portlet Bridge is then responsible for retaining that "state" within JSF during every Render or re Render of that page until another ActionRequest is made against that portlet. This is required because portals can call Render on a portlet many times without that portlet having been involved with user interaction, and this ensures that the content of that portlet does not alter during each of those Render calls.
If you'd like to see functionality to disable this spec compliant feature, then please raise a JIRA for it, and maybe contribute a pull request that would fix it.