because you are not using the tool in the correct manner.
In essence, you are using a screw driver to hammer a nail.
you are using form builder to edit a form. It will not work. and is not the intended use of the tool. you are not getting the desired results because you are using the tool to do something it is not intended to do.
you say yourself that if you use it correctly to create a new form it works. that is the intended job of the tool.
like i have already said, if you already have the form created, form builder is not the correct tool.
you need to manually add the functionality like persistence and validation using those behaviors, not by using the form builder wizard.