Repro

• Build the "Menus Example" (widget-based)
• Run on Windows-10 with Narrator or NVDA screen reader
• Alt+E (brings up edit menu)
• Navigate to Format with arrow keys
• Menu is read as "Format Alt+F" (NVDA) or "Format Menuitem Menu Key Alt+F" (Narrator), but no indication is given that there is a submenu

Other data

Compare with notepad.exe, View/Zoom, which will read "Zoom Submenu Z" with NVDA or "Zoom Menuitem Collapsed Menu Key Z" with Narrator. The word "submenu" or "collapsed" give hint that this opens a submenu. 

On macOS, this "Menu Example" works correctly with VoiceOver, it will add "submenu" when it reads menu items that show a submenu.

Didn't try Linux, but did find this existing issue that might be related: https://bugreports.qt.io/browse/QTBUG-40858

Also see this problem with Qt Creator on Windows (the Qt Creator program menus).