Not all entries of large submenu visible on Wayland

Originally reported for LibreOffice ( https://bugs.documentfoundation.org/show_bug.cgi?id=167034 ), but this is also reproducible with the simple Qt sample program attached here.

1) run KDE Plasma Wayland with a taskbar at the bottom (default setup)

2) compile and run the attached sample program ( main.cpp ) on a screen that has a taskbar at  the bottom

3) long-press the toolbutton to open its menu

4) Hover over the "Submenu" entry to open the submenu

Actual result:

Some submenu entries (in my case "Entry 0" and "Entry 48") are off-screen (not visible, not clickable).
Screenshot:

Expected result:

All submenu entries should be on-screen (visible, clickable).

More information:

1. This works fine when forcing the app to run using the xcb platform backend/on XWayland using QT_QPA_PLATFORM=xcb, Screenshot:
2. It also works fine for me when I run the app on an external monitor that doesn't have a taskbar.

In case 1), the menu also overlaps the taskbar, in case 2), there is no taskbar. Is this maybe some mismatch between the menu size being calculated using the screen height, but then not actually using that full height because of the taskbar on Wayland?