I've searched far and wide for an answer here and have never run into this issue - my HubSpot form, that lives on a HubSpot landing page, is ONLY submitting user information when in incognito mode. When in a regular browser, nothing happens when clicking the submit button, but it seems to work just fine in incognito mode. I've checked the backend of the page and form and nothing seems to be out of the ordinary.
We had a similar problem with a client, the HS form did not work in normal mode, but it worked in incognito mode.
Our problem was different in that we were using WordPress and a Hubspot embed form. What we found, was a javascript conflict was blocking the Hubspot form.
So I was wondering are you using any other Javascript on the web page?
If so I recommend try removing it and see if that resolve the problem.
@Crocodilemarket - No, actually. No Javascript has been added to the backend at all. That was one of the things I first looked for when trying to figure this out on my own before posting here.
@Crocodilemarket - I just received the submission so I think you might be right! Not sure why I didn't think about cache clearing beforehand - thank you!
Additional question: did you get the inline "thank you" message after submitting the form?