empty icons on QQC2 and Qt 5.12

on Linux, for most styles (for instance Material is affected) the following code, doesn't show an icon anymore (worked fine in Qt 5.10 and 5.11):

Button

looking into QQuickIconImage::setName(), it loads the icon with

d->icon = QIconLoader::instance()->loadIcon(name);

if i do:

qDebug()<<QIconLoader::instance()->themeName(); i can see it has an empty theme name, instead of the system one.

If i add before loadIcon() the code:

QIconLoader::instance()->updateSystemTheme();

it sets the correct theme and the icon is loaded correctly.

updateSystemTheme doesn't seem to be called anywhere in either qtbase or qtquickcontrols2 repositories, where should this be done?