Turns out this is a known bug:
https://bugreports.qt.io/browse/QTBUG-98483
and
https://bugreports.qt.io/browse/QTBUG-103212
That they have already fixed in Qt 6.2 and later but for some reason refuse to fix in Qt 5.15.x and earlier until Qt 5.15.14.
And of course are hiding that fix until a full 12 months have passed for that release.
Sorry, but Qt is really being an ass about things now.