SVG QIcon rendered to SVG gets rasterized

I want to export a table/tree/... widget to SVG and use QWidget::render for that on a painter on a QSvgGenerator. Many parts are rendered nicely scalable but not so the expanded icon. Here is a screenshot from the SVG as opened in Inkscape:

So, my idea was to replace the standard icon with a SVG icon but I found out that QIcon does in no way support rendering to vector scalable outputs. My source codes are attached. Here is the zoomed screenshot of the exported SVG when the expand icon was replaced with a circle and another QIcon was rendered below Child 1:

This is what I would expect the zoomed output in inkscpae to look like: