The same thing is happening to me. I’ve just installed WordPress 5.4.
I’ve opened the developer tools and I get this error after clicking Publish:
Access to fetch at 'https://<my_site_url>/wp-json/wp/v2/pages/33/autosaves?_locale=user' from origin 'https://<my_site_url>' has been blocked by CORS policy: Request header field x-wp-nonce is not allowed by Access-Control-Allow-Headers in preflight response.
