On PlasmaComponents version 2 we use property iconName for Button, but for PlasmaComponents3 we should use icon.name property. Nothing is said about this in the documentation on this page: https://api.kde.org/frameworks/plasma-framework/html/classorg_1_1kde_1_1plasma_1_1components_1_1Button.html
That is the documentation for the PC2 button, JFYI