You may have the form set to a different follow-up page in the landing page itself. You can check this by going into the page, and editing the form. You'll be given three options for the redirect, Form Default, Custom URL, and stay on page. You may have a setting different from form default, which is why you're seeing this result.
On the landing page, edit the form using the gear and make sure that the follow-up page is formed-defined, not defined on the landing page. If you have checked the forms 2.0 editor to make sure that you have defined the thank you page as your follow-up page after submitting, this is likely the issue.
That was it!! Thank you so much! I would have spent hours figuring that out. But I knew it was something I was missing.