KXmlGui ignores Action attributes enumerated at https://invent.kde.org/frameworks/kxmlgui/-/blob/master/src/kxmlgui.xsd beyond "name". For example, setting "text", "icon", or "group" in the ui.rc file are ignored. Creating the Action and setting the "name" in the XML file and then setting the "text", "icon", or "group" attributes programaticaly in C++ works as expected.
As a followup piece of information, setting attributes like "icon" work correctly for Menus. So, this appears to just be a problem with Actions.