The context menu event created on X11 systems when pressing the menu key does not contain the keyboard modifiers, the offending code is in qkeymapper_x11.cpp, in the QKeyMapper::sendKeyEvent() function and looks like

QContextMenuEvent e(QContextMenuEvent::Keyboard, pos, globalPos);

but should instead read

QContextMenuEvent e(QContextMenuEvent::Keyboard, pos, globalPos, modifiers);