Here’s the same issue with a correct fix: https://community.hubspot.com/t5/Lead-Capture-Tools/Solved-HubSpot-form-not-redirecting-to-HubSpot-page/td-p/623536
"There are two places with options which control what happens after submitting a form. One of them is within the forms editor, the other is inside of the landing page editor.
Can you check in your landing page editor; click on the forms element on the landing page. A left sidebar should open and when you scroll down, you’ll also see redirect options. This should also have your redirect URL from above."