I got two different failure modes. First it displayed an error message (which appeared instantly, and was caused by some JS error in the page which caused it to not submit the form at all), and then a while later the same behaviour as you, but devtools showed a 500 error from the backend.
Oh shoot... I submitted that form too, and I wasn't clear if it failed or not. It said "you'll hear from us soon" but all the fields were still filled and the page didn't change. I gave them the benefit of the doubt and assumed it submitted instead of refilling it...