Inconsistent QComboBox button highlight behaviour

The highlight status of the dropdown button next to a combobox is not consistent.

Minimal code:

#include <QApplication>
#include <QComboBox>
#include <QMainWindow>
#include <QVBoxLayout>

class TestWindow : public QMainWindow
{
public:
    TestWindow(QWidget *parent = nullptr) : QMainWindow(parent)
    {
        QComboBox* cb = new QComboBox;
        cb->addItem( "One" );
        cb->addItem( "Two" );
        cb->addItem( "Three" );

        QVBoxLayout* l = new QVBoxLayout;
        l->addWidget(cb);
        l->addWidget(cb);

        QWidget* w = new QWidget;
        w->setLayout(l);
        setCentralWidget(w);
    }
};

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    TestWindow w;
    w.show();
    return a.exec();
}

Steps to reproduce:

• Launch the application
• (Notice the dropdown button is blue highlighted)
• Click outside the application
• (Notice the dropdown button is not blue highlighted)
• Click inside the application
• (Notice the dropdown button is blue highlighted)

Now also:

• Click on the dropdown button so the list of options appears
• Click outside the application while the list is open
• (Notice the dropdown button is not blue highlighted)
• Click inside the application
• (Notice the dropdown button is not blue highlighted)

The blue highlighting of the dropdown button of a combo box  is not consistent when bringing the focus to the parent window depending on whether the combo box list was open or not when the parent window lost focus.

Attached Capture1.png and Capture2.png show the different highlight states.