If a QMenu is styled with the fllowing CSS code from the Qt documentation https://doc.qt.io/qt-5/stylesheet-examples.html then it looks good for all Qt Versions from Qt 4.0 - Qt 5.11.

 
QMenu {
    background-color: white;
    margin: 2px; /* some spacing around the menu */
}

QMenu::item  {
    padding: 2px 25px 2px 20px;
    border: 1px solid transparent; /* reserve space for selection border */
}

With the current Qt version 5.12.2 this is broken:

The picture shows that the styling is broken for checkable QActions with an Icon. If an Action with an Icon is checkable, then the menu should show the Icon but now the menu shows the checkmark plus the icon.

The attached test case shows this problem.

While this looks like a "Somewhat Important" bug that will never be fixed like any other "Somewhat Important" bug, it is a little bit more serious because I have not found any workaround for this problem so far. That means, any Qt application that uses stylesheets seriously (like our QmixElements software) is affected by this problem.