Have greek keyboard layout and an English Keyboard.

1> Run the attached test app.
2> Press, ;a ;e ;υ ;i to get, ά έ ύ ί respectively.
3> Hover mouse over both Menu's
4> Repeat step 2 to see the output as ´α ´ε ´υ ´ι respectively.

Note that this is not reproducible on Windows and also not repro with Qt5.

Possibly this used XIM , which was replaced in Qt5 ,
https://codereview.qt-project.org/#change,51555

Also note that, you need to hover over both the menus.