I was informed at bugreports.qt.io/browse/QTBUG-136715 that:
It seems to be an issue specific to Fedora, which is not a supported platform.
Considering that the KDE Workstation Spin was recently promoted to an Edition, this seems like an accident on Red Hat’s part, since, as opposed to openSUSE (and Red Hat Linux!), this isn’t a particularly useful Qt development platform if platform-specific bugs aren’t investigatable by upstream. I realise RHBZ should be for this, but…
Qt upstream makes their own decisions independent from us. They do not even support the latest openSUSE Leap nor do they support openSUSE Tumbleweed. They do support Red Hat Enterprise Linux, though.
I found this document https://doc.qt.io/qt-6/linux.html
Basically only RHELs and other enterprise platforms are supported it seems.
But note that the page does not acknowledge Wayland support at all.
I expect that you would be better off raising a bug on the KDE bug tracker.
I would expect them to know how to raise issues with Qt devs.
I did try running your code, I can resize the main window from any corner on my KDE plasma desktop. Is that what you are reporting does not work?
They just file them the same way. They use CPP though, so it’s easier. In retrospect, it’s PySide that’s officially endorsed, not PyQt, so I suppose I should have gone through them instead. 
If you look at the attached screencast, the vertical resize direction is inverted when resizing from solely the top-right corner.
Managed to create a reproduction in CPP, but I’ll go through bugzilla.redhat.com/enter_bug.cgi?component=python-pyqt6&product=Fedora next time, since upstream PyQt6 provides no bug tracker; solely a mailing list! 
It’s a very responsive mailing list. My experience is that bugs are fixing quickly.
Thanks. However, although I do utilise e-mail (and Atom) for my notifications, it’s not how I’d want to interact to file a bug. The immutability of responses and already-posted attachments would mean a deluge of noise every time someone wants to change something. That’s a little more friction than I’m willing to deal with.
Given that PyQt is a wrapper around the C++ Qt getting your C++ version fixed is likely the best route to a solution.