Permanently show Custom Fields in Editor

this should work but you overwrite the values for existing objects.

to add default values only for new objects try that :

$postType = "tender";

add_action( 'save_post_' . $postType, function ($post_ID, \WP_Post $tender, $update) {

    if (!$update) {
        // default value for new object

        // Add the meta data you want the custom post type to have
        $tender_meta_data = [
            'briefing',
            'closing',
            'rfirfqnumber',
            'download',
        ];

        foreach ( $tender_meta_data as $meta ) {
            add_post_meta( $tender->ID, $meta, '', true );
        }


        return;
    }



}, 10, 3);