Wrong result in fetching User meta of WordPress

I have custom post type Task where I assigned Users as Volunteers.

Now I would like to fetch how many tasks assigned to a Volunteer.

I am using below code.

$users = new WP_User_Query(array(
        'meta_query' = array(
            'relation' = 'OR',
            array(
                'key' = 'last_name',
                'value' = $_POST['search'],
                'compare' = 'LIKE',
            ),
        ),
    ));
    $users_found = $users-get_results();


    $names = array();
    foreach ($users_found as $user) {
        $this_users_tasks = get_user_meta($user-ID,'task',false);
        $names[$user-ID] = $user-display_name.' ('.count($this_users_tasks).')';
    }

    
    echo json_encode($names);
    die;

But I am getting wrong result.