This seems to be pretty serious. Words used in #plasma were 'blacklist qt-5.9.2', 'huge breakage', 'wow'...
KDE distributions mail suggesting to revert the commit introducing the regression: https://mail.kde.org/pipermail/distributions/2017-October/000253.html
Maybe solved in 5.9.3, if the last comment in QTBUG-64017 doesn't mean more trouble.
Let's consider this fixed by Qt-5.9.3.