The problem has nothing to do with the form itself nor with standard Marketo forms behavior.
Somebody who has no idea how Marketo forms work -- like literally, this is some of the worst code I've ever seen -- tried to override the forms library logic with some failed fancy-pants behavior. That person needs to fix their code or remove it. You aren't using anything close to a regular setup here.
What Sanford means to say is that there's a big block of jquery code in your landing page that's modifying the natural behaviour of your Marketo form. Your opening post makes it sound like Marketo forms in your instance is at fault for the abnormal behaviour but this is not the case.