qml binding to a QML_EXTENDED property doesn't update when changed signal emitted

// existing public C++ class for widget applications etc.: no QML API goes here
class Q_PDF_EXPORT QPdfDocument : public QObject
{
    Q_OBJECT
    Q_PROPERTY(int pageCount READ pageCount NOTIFY pageCountChanged FINAL)

public:
    int pageCount() const;
...
Q_SIGNALS:
   void pageCountChanged(int pageCount);
}

// now we expose a QML wrapper, without subclassing
class Q_PDFQUICK_EXPORT QQuickPdfDocument : public QObject, public QQmlParserStatus
{
    Q_OBJECT
    Q_INTERFACES(QQmlParserStatus)

    Q_PROPERTY ... (several added ones)

    QML_NAMED_ELEMENT(PdfDocument)
    QML_EXTENDED(QPdfDocument)
    QML_ADDED_IN_VERSION(5, 15)
public:
    ...
private:
    QPdfDocument m_doc;
}

$ cd qtwebengine/tests/manual/quick/pdf
$ qml withdoc.qml