Note the default behavior is for redirection to the AMP version to happen as early as possible when mobile visitors access the non-AMP version. By default this redirection is performed in JavaScript, meaning it is expected for the non-AMP version to display briefly. However, the redirection logic happens in the head
and should stop the page from loading before redirection happens.
You can avoid the client-side redirection logic and use server-side redirection logic via PHP instead. You can do so by adding this PHP code to a custom plugin:
add_filter( 'amp_mobile_client_side_redirection', '__return_false' );
The reason why this is not the default is that client-side redirection is more reliable across the wide variety of WordPress hosting environments. If you have page caching (which you should), you’ll need to make sure that the cache is varied by mobile device in the same way as it is detected by the AMP plugin. For more, see §Client-side vs Server-side Redirection.
Nevertheless, please still share the URL for your site because the non-AMP version should not be getting displayed prior to client-side redirection.