I used github "blame" on the qwebengine file in question and even though the change that is causing us issues has been in place for over 5 months, it was never included in any of the Qt 5.13.x, or 5.14.x releases.
Why on earth would you wait to release a change like this into a "LTS" release and never put it in any of the earlier dev series releases? It is almost like Qt wants to break things with this type of strategy. Or possibly it wants to hold out on "fixes" to so they are only available to commercial LTS customers first (or not at all!).
See
https://github.com/qt/qtwebengine/bl...t_qt.cpp#L1147
And then check the 5.13, and 5.14 branches and see that these changes are not in them at all.