QFontMetrics::horizontalAdvance() is wrong when the initial point size of the font is -1

Question

When constructing a QFont instance f with the default point size of -1 (doc says the point size of the font is set to a system-dependent default value), QFontMetrics {f}.horizontalAdvance() returns a value which is greater than what I expect.

Here's a complete demo:

class QTextWidthTest :
    public QWidget
{
public:
    QTextWidthTest()
    {
        this->setFixedSize(600, 400);
    }

private:
    void _paintExample(QPainter& painter, const int pointSize,
                       const QString& text) const
    {
        const QFont font {"sans-serif", pointSize};

        // draw text
        painter.setFont(font);
        painter.setPen(QColor {"black"});
        painter.drawText(0, 0, text);

        // draw horizontal advance
        painter.setPen(QPen {QColor {"red"}, 2});
        painter.drawLine(0, 3, QFontMetricsF {font}.horizontalAdvance(text), 3);
    }

    void paintEvent(QPaintEvent *) override
    {
        QPainter painter {this};
        const auto sysPtSize = QApplication::font().pointSize();

        painter.translate(50, 50);
        this->_paintExample(painter, sysPtSize, "meow");
        painter.translate(0, 50);
        this->_paintExample(painter, sysPtSize, "hello world!");
        painter.translate(0, 50);
        this->_paintExample(painter, sysPtSize, "hello how are you today?");
        painter.translate(0, 100);
        this->_paintExample(painter, -1, "meow");
        painter.translate(0, 50);
        this->_paintExample(painter, -1, "hello world!");
        painter.translate(0, 50);
        this->_paintExample(painter, -1, "hello how are you today?");
    }
};

Here's the result:

Is this a known behaviour or a known bug?