there's no solution for that. It seems as normal behavior of Goutte/Guzzle. As a workaround we ended up with following solution: in case Goutte driver is used we check page contents for all <textarea>
elements, and if any found then for each we get their contents and plainly reinsert as follows:
$elements = $this->pageContent->findAll('xpath', '//textarea');
foreach ($elements as $element) {
$element->setValue($element->getText());
}