Hi,
There's a long-standing bug with KDE/Qt apps under Wayland having their popup menus offset in certain multi monitor configurations.
Now, there has been a fix for this, which has been applied to 6.4, see
https://bugreports.qt.io/browse/QTBUG-85297
It looks like this patch can be applied to 5.15 as well.
It might be worth investigating if this patch can be shipped with Qt5 as well?
grtz,
Sander