it's not actually a bug.
the initial code dynamically sets the forms action to post to the page name.
The code change i supplied sets the forms action dynamically to the page name, but also includes any query string variables.
You dont always need to, or even want to, pass the query string variables used to load the page, in the case of a plugin page, it is needed.
but really, you should only include querystring variables with the forms action on an as needed basis.