I had a ton of issues with QT menus until the latest patches from KDE hit Arch's QT packages. Fixed them
https://bugreports.qt.io/browse/QTBUG-87303
https://bugreports.qt.io/browse/QTBUG-87332
If KDE patches actually make menus work okay, that's great. I've been wondering how KDE deals with the bugginess of the wayland QPA, turns out the answer's simple and not KDE-exclusive, nice.
upd: seems like no popup constraint adjustment is set on the kde/5.15 branch. Hopefully there are other improvements though…