Longterm cache clear via backend not working

Use this simple test script in your QUIQQER root:

<?php

define('QUIQQER_SYSTEM', true);

require_once 'bootstrap.php';

ini_set('display_errors', 1);
error_reporting(E_ALL);

\QUI\Cache\LongTermCache::set('peat_test', 'Pferd');

try {
    \QUI\Cache\LongTermCache::get('peat_test');
    echo "in cache";
} catch (\Exception $Exception) {
    echo "not in cache";
}

1. Execute script once (output "in cache")
2. Comment out the LongTermCache::set - Line
3. Go to QUIQQER backend and click "Clear longerm cache" in system settings
4. Execute script again (output is still "in cache" but expected was "not in cache")