Qt5 has never respected the QStyle::SH_DialogButtonBox_ButtonsHaveIcons setting correctly in all conditions, in my experience, at least not for QPushButtons:

• on Linux, they always seem to show icons, even on a KDE deskop configured NOT to show them.
• on OS X, they will not show icons as expected, at least not when using the Cocoa platform theme. Enable the KDE platform theme from KF5-frameworkintegration, and they will show icons even if the KF5 settings tell otherwise.

I can see at least 1 bug in qpushbutton.cpp: QPushButton::sizeHint() should check for `(!icon().isNull() && showButtonBoxIcons)` instead of `(!icon().isNull() || showButtonBoxIcons)`.

This is not enough, however, as shown by the screenshot I'll be attaching.
It was made on a KDE4 desktop with Qt 5.5.1 and KF5 installed in /opt/local; KF5-frameworkintegration was deactivated so as to be sure that it is not the source of this bug.
The test application is kdebugdialog5, which depends on kf5-kdelibs4support ; it is possible that this framework is involved through its KPushButton class.
I added a qDebug statement to QPushButton::sizeHint; using the Fusion style it shows that the style indeed hints that no icons should be shown on buttons.