Qt::LayoutDirection is not inherited by objects children. When Qt::LayoutDirection is set on a QMainWindow submenus does not inherit the Layoutdirection. This causes menus to i.e. turn one way, while the submenus go the other way.

Update: If you set the layout direction on QApplication instead, this behaves as expected. The problem is that the menus are not created yet at the point where the layout direction is set on the mainwindow, and that widgets don't inherit the layout direction when created with a parent or reparented.