form builder is a tool for Creating forms and should not be used to edit them. even if the for was originally created with form builder, it should not be used to make edits to the form. especially if you made modifications to the form after creating it. Form builder has no way of knowing what modifications you may have made t retain them.
"editing" the form is basically the same as creating it again from scratch. there are simply to many things you can do to modify a form after it is created for form builder to be able to maintain the changes on editing the form.
Use form builder only for the initial building of the form, not for editing it afterwards.