QWebEngineView and QWebEngineView::loadFinished() (from your subject and first post) and your code using QNetworkAccessManager directly are totally unreleated related to each other.

If you are actually using QWebEngineView then you can get the HTML from the view's QWebPage through its toHtml() method.

If you fetching the web URL using QNetworkAcessManage and QNetworkRequest:
There is more than one reason a QNetworkReply might finish with an empty response. Errors are one. Have you checked for those? A deliberately empty response is another.