Hi Akram,
Sorry to make you feel like that. What I meant is that's the default behavior of the fields.
You still can change the behavior with the plugin action, like this:
add_action( 'rwmb_after_save_post', function( $post_id ) {
$group = get_post_meta( $post_id, 'group_id', true );
$terms = $group['terms'] ?? [];
wp_set_post_terms( $post_id, $terms, 'your_taxonomy' );
} );
