When a form submission creates a new contact, and when it updates an old one

Short answer

In the updated form editor, Settings > General has Automatically create new contacts from unknown email addresses. The legacy editor's equivalent, per HubSpot's KB, is Always create new contact for email address, which creates a new contact for every new email "regardless of any matching usertokens" from the browser cookie. Without it, a new email from a known browser can update the cookie's existing contact.