If setEnabled(false) is called on sub menu it has no effect, item remain clickable. if it is called on any other individual menu item it works.

It can be easily reproduced with menu example by adding one line
formatMenu = editMenu->addMenu(tr("&Format"));
formatMenu->setEnabled(false);
Attached two screenshots shows 5.6.1 not working but earlier 5.6.0 working.
Bisecting pointed to this commit which seems introduced this
https://codereview.qt-project.org/#/c/149727/