As a result OutputCache puts in cache file boolean result of buffering.

public function end()
{
    $key = array_pop($this->keyStack);
    if ($key === null) {
        throw new Exception\RuntimeException('Output cache not started');
    }
    
    $output = ob_end_flush(); // boolean
    if ($output === false) {
        throw new Exception\RuntimeException('Output buffering not active');
    }

    return $this->getOptions()->getStorage()->setItem($key, $output);
}

Please use {color:green}ob_get_flush(){color} instead {color:red}ob_end_flush(){color}, which returns result of buffering as a string.