P2: Important Description
it was working just fine in the qt 5.15.
ive found out that your example also doesnt work so its not a bug on our side.
the example can be found here: https://code.qt.io/cgit/qt/qtwebengine.git/tree/examples/webenginewidgets/notifications?h=6.2
i dont know if it's related to this: QTBUG-96525 or not.
as a workaround i am reading manually file(QFile) from qrc and doing setHtml() with baseurl param as normal qrc url.
thankfully someone reported that QTBUG-96525 so it can read .js files from that manually set html-code.
it works - as workaround till you fix it.