webengine = new QWebEngineView();
webengine->page()->setZoomFactor(10);
qDebug() << webengine->page()->zoomFactor(); // prints 1

Edit:
If I wait until the page has loaded before calling setZoomFactor, and use a value of 5 or less, zoomFactor appears to get the correct value. However, it still has no effect on the output.