border-radius does not clip background and does not draw border in popup modus

I have a QFrame that is used as a drop-down panel, and which has the Qt::Popup flag set. When setting a border-radius, the background is not clipped properly.

The stylesheet:
QFrame

{ border: 1px solid green; background-color: red; border-top-right-radius: 0; border-top-left-radius: 0; border-bottom-right-radius: 5px; border-bottom-left-radius: 5px; }

The attachment shows the bottom corners. I also tried setting `border-radius: 5px` to set a radius on all corners. Then the bottom corners look the same, and the top corners look as shown in the attachments. It seems the top right corner is the only location where an actual rounded border is drawn, allthough still the background is not clipped.

When using rounded borders on 'normal' widgets in the UI, everything is oke. It is only on this frame, that has the Qt::Popup flag set, and which is continuously hidden and shown, where the problem occurs.