You must have other code on the page that is causing the form button to disappear. This is relatively common in some scripts to prevent someone from clicking a button twice and double-submitting a form. You would either have to remove that code, or add a check to make sure the form is valid before hiding the button. This isn't webassist code, so I'd need a copy of the page to debug and determine where and how the code is initiated.
Since it is client-side code, I would just need a live URL to see the page and debug it.