Hi Martin,
The custom field with the ID post_title
only works on the frontend (just a trick). On the backend, WordPress has the main field (pre-built) post_title
so it does not show the custom field with the same ID.
You can also follow this solution to only use the custom field and remove the main field Post Title https://docs.metabox.io/save-wysiwyg-content-post-content/.
Add this code to the file functions.php or use the plugin Code Snippets.
add_action( 'init', function () {
remove_post_type_support( 'post', 'title' );
} );