On Mac OS 12.5.1, corner widget in QScrollArea is not drawn correctly

Hi,

On Mac OS 12.5.1, using Qt 6.4.0, the corner widget of a QScrollArea is not drawn correctly. The pictures attached show a default QWidgets app with a QMainWindow where the constructor of the main window contains the following:

ui->setupUi(this);
QScrollArea * sa = new QScrollArea;
sa->horizontalScrollBar()->setRange(0, 2000);
sa->verticalScrollBar()->setRange(0, 2000);
setCentralWidget(sa);

 In one picture, there is a status bar in the QMainWindow (default for the app as created by QtCreator). The corner widget is a strangely styled square. In the second, the status bar has been removed and the the corner widget is still a square "underneath" the curvature of the window. The final picture is from google chrome, showing the expected appearance. 

added in edit - I noticed this in using QAbstractScrollArea, where it also happens, hence the direct manipulation of scrollbar ranges, which I guess is not the normal way of using QScrollArea?

Cheers, best wishes,

Mick