I don't even need to dig through the paint loop. I've written QT software, and you can do it yourself.

Add a QButton with no styling, you will have a standard button widget in macOS and Windows (or a KDE styled one in Linux). Now have an event that modifies the button in some way (adding a CSS style to it is common, but also changing the sizing manually will usually do it) and the style of the button will immediately change to a QT-style one. The difference is so obvious in Windows and macOS, that it would be laughable to claim otherwise; but you can also just ask the QT Group devs or the community, they've reiterated/experienced this behavior plenty of times.