Order of search results based off of a backend custom field and also in alpha order

Surname is already stored as user meta last_name so you don’t need a custom field. You do need to make sure you’re only affecting the page you want to.

    function sort_users_by_lastname( $query ) {
        // only if we're on users.php
        // and when orderby isn't changed. the default (when nothing is set) is "login"
        $screen = get_current_screen();
        if ( $screen->base == 'users' && is_admin() && $query->get('orderby') === 'login') {

            // Set the orderby parameter to user's last name
            $query->set( 'meta_key', 'last_name' );
            $query->set( 'orderby', 'meta_value' );
            $query->set( 'order', 'ASC' );
        }
    }
    add_action( 'pre_get_users', 'sort_users_by_lastname' );