FWIW, qtwebkit does get some maintenance by Annulen. Bugs reported there are still being responded to. We now build our webkit based on annulen's version 5.212 with the following patches:
See qtwebkit _*.patch at:
https://github.com/Sigil-Ebook/Sigil/tree/master/docs
None of these effect fonts particularly, but Qt (not webkit) has serious issues with fonts especially on Windows machines due to use of the OS2 font tables incorrectly:
See:
https://bugreports.qt.io/browse/QTBUG-73660
and the following font umbrella issue:
https://bugreports.qt.io/browse/QTBUG-74860
So the bug could well be in Qt itself versus QtWebkit.