Field has disappeared on post page

To expose a postmeta field in the Block Editor, you need to register it and enable it to show in the REST API.

With your field being named “manufacturer_url” you would use something like this:

<?php
// Use "init" hook
add_action('init', 'wpse_register_mfr_postmeta');
function wpse_register_mfr_postmeta() {
    // Make sure to set your CPT slug as the first argument
    register_post_meta('mycptslug', 'manufacturer_url', array(
        // Critical part: show in REST API
        'show_in_rest' => true,
        // You're probably storing a string but double-check first:
        'type' => 'string',
        // If you only allow 1 manufacturer_url per post, use this
        'single' => true
    ));
}
?>

You can place this in a custom or child theme’s “functions.php” file, or as its own custom plugin just by adding a comment with “Plugin Name” at the top. (Since you probably don’t want the data disappearing if you switch themes, setting it up as a plugin is likely the best approach.)