QProxyStyle for QPushButton works strange

Using a QProxyStyle with custom style sheets for QPushButton works strange. See the attached images, the first one (style.png) is without QProxyStyle. Both examples uses simple style sheet:
QWidget {
background: #535353;
font: 12px Arial;
color: #FFFFFF;
}

Widgets in the second example (proxy.png) are using this dummy proxy-style:
class TestStyle : public QProxyStyle {};
The QLabel is rendered correctly, but the buttons are wrong (rendered without style sheet?)