The form submits, but the AJAX request to the REST endpoint never reaches the Contact Form 7 controller. The server returns a 404 status code, so the browser displays the site’s Not Found page instead of the success message.
This happens when the URL /wp-json/contact-form-7/v1/contact-forms/ID/ is blocked, rewritten, or never reaches WordPress. Server rewrite rules, security filters, or missing rewrite rules are typical culprits.
When the request fails, client‑side validation stops. Required‑field warnings do not appear and the form appears to disappear after clicking Submit.
The shortcode may still render on the page, but the underlying form resource cannot be located. WordPress treats the request as a missing resource and sends a 404 response.