@paul2009 and @humxahafeex, actually there was a way to fix it and redirect from within the iFrame. The following piece of code was provided by the forms website (I replaced the form ID with --- in the code here). It doesn't seem to always work, but it sometimes does. I'm not sure yet why it's still inconsistent.
<div formsappId="---"></div>
<script src="https://my.forms.app/static/iframe.js" type="text/javascript"></script>
<script>
new formsapp('---', {
width: '100%',
height: 'formHeight'
});
</script>