QFileDialog has a black area on macOS 10.13.5

Native file dialogs invoked via QFileDialog often have a black area on the side (see attached screenshot, please ignore the red rectangle), which seems to be related to the presence of other widgets in the window below. For example, if I trigger this dialog from a window with a QWidget in it, it is rendered correctly. But, if I trigger the dialog from a window with a QGraphicsView, the bug occurs.

With this code example it replicates about 95% of the time for me:

#include <QApplication>
#include <QFileDialog>
#include <QGraphicsView>
#include <QMainWindow>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QMainWindow w;
    w.setCentralWidget(new QGraphicsView());
    w.show();

    QFileDialog exportDialog;
    exportDialog.setAcceptMode(QFileDialog::AcceptMode::AcceptSave);
    exportDialog.exec();

    return a.exec();
}