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.