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);