Phalcon\Mvc\View: sometimes getRender returns empty string with the same params

I wrote the following peice of code to localize the problem. It executes getRender 100 times and groups result string length. output examples: Array ( [510] => 100 ) Array ( [510] => 93 [0] => 7 ) Array ( [510] => 99 [0] => 1 ) Array ( [510] => 87 [0] => 13 ) ... etc

so, sometimes getRender returns empty string with the same params

    $vars = array(
        'title' => 'title1',
        'html' => '<div>test</div>',
        'id' => 'video0',
        'first' => 1,
    );
    $view = $this->getDI()->get('view');
    $variants = [];
    for ($i=0; $i < 100; $i++)
    { 
        $view->setVars($vars);
        $html =  $view->getRender('article', 'video', $vars);
        $idx = mb_strlen( $html );
        if ( !isset($variants[$idx]) )
        {
            $variants[$idx] = 1;
        } else {
            $variants[$idx] = $variants[$idx] + 1;
        };
    };
    print_r($variants);

view service initialization:

    $view = new \Phalcon\Mvc\View();

    $config = $di->get('config');
    $view->setViewsDir($config->application->viewsDir);

    $view->registerEngines(array(
        '.volt' => function ($view, $di) use ($config) {

            $volt = new \Phalcon\Mvc\View\Engine\Volt($view, $di);

            $volt->setOptions(array(
                'compiledPath' => $config->application->cacheDir,
                'compiledSeparator' => '_',
                'compileAlways' => $config->alwaysCompileTemplates,
            ));

            \fx\view\FunctionsRegistrar::register($volt->getCompiler());

            return $volt;
        },
    ));

    return $view;

We tried to change phalcon version from 2.0.13 to latest with no effect.