Unexpected and inconsistent overload resolution when calling C++ from QML

Consider the following C++ class:

class TestThing : public QObject
{
    Q_OBJECT
    QML_ELEMENT
    QML_SINGLETON
public:
    explicit TestThing(QObject *parent = nullptr);
    Q_INVOKABLE void foo(const QString &bla);
    Q_INVOKABLE void foo(TestThing *a);
}; 

And the QML code

Item {
    property QtObject obj: TestThing;
    Component.onCompleted: TestThing.foo(obj)
} 

Full code at https://invent.kde.org/nicolasfella/qmloverload

The QML code will call the QString overload instead of the TestThing * overload, which I find rather unexpected.

Changing the type of the obj property from QtObject to TestThing makes it choose the TestThing overload (as I expect).

What's even more confusing is that when I remove the declarative type registration macros and rely on runtime registration then the behavior changes and the TestThing overload is chosen.

I found this behavior in KDE Plasma after porting to declarative type registration cause a regression.

Qt 6.6

qtbase f4b91d595ce0ec69b01848bfe68d468c63d20d5a

qtdeclarative e1fbca3d7338c63672af7224e6b70ad11aea9332