SVG icons are stretched (not drawn pixel-perfect) with display scaling enabled

[Violet Giraffe]

I'm simply setting a QAction icon from SVG file, and I emulate display scaling with setting env var QT_SCALE_FACTOR to 2.0.
The icon looks like this, obviously rendered at 1x and then stretched. How can I work around this? Why does Qt do this in the first place with HiDpi support enabled?
Tested with Qt 5.15.2.

Here's the complete repro code:

int main(int argc, char *argv[]) {
	qputenv("QT_SCALE_FACTOR", "2.0");
	QApplication::setAttribute(Qt::AA_EnableHighDpiScaling, true);
        QApplication::setAttribute(Qt::AA_UseHighDpiPixmaps, true);
	QApplication a(argc, argv);
	a.setStyleSheet("*{background: #191919;	color: #DDDDDD;}");
	QMainWindow w;
	QToolBar* tb = new QToolBar;
	w.addToolBar(tb);

	QPixmap icon("help.svg");
	icon.setDevicePixelRatio(2.0); // This appears to change nothing
	tb->addAction(new QAction(icon, "text"));
	w.show();
	return a.exec();
}

Here's the sample project - an SVG icon and the code above. Since I can't attach a ZIP directly, here's a link to cloud storage. Turn off shadow build for the icon to load successfully.

[Violet Giraffe]

Turns out that creating icons via QPixmap is an anti-pattern. It is drawn correctly if constructing QIcon directly: QIcon icon("icon.svg");