Although ui_standards.rc is embedded into the library (according to https://cgit.kde.org/kxmlgui.git/tree/src/kxmlgui.qrc) it is not found by an application (tried with KF5 umbrello portable package from https://software.opensuse.org/package/mingw32-umbrello5-portable?search_term=mingw32-umbrello5-portable). As fallback there is a file ui_standards.rc installed in <install-root>/etc/xdg/ui/ui_standards.rc, but on starting an application Qt searches this file in <install-root>/bin/xdg/ui/ui_standards.rc. I don't think that is was a good decision by Qt to place all files below bin while the rest of oss packages uses <install-root>/share for data. This could be partial corrected by using a qt.conf file which is unfortunally not used by QStandardPath. It puts an additional burden on package maintainer and porter beside the huge work to make all this Qt5/KF5 changes working on Windows.
A workaround was added to https://build.opensuse.org/package/show/home:rhabacker:branches:windows:mingw:win32:KF565/mingw32-kxmlgui?rev=5 and the issue has been fixed by the update of Qt to 5.11 (see https://build.opensuse.org/package/show/windows:mingw:win32/mingw32-libqt5-qtbase?rev=59)