Error: Unable to determine callable overload (calling slot in subclass)

If you have a "base" QApplication class where the object name "MyApp" is registered with qmlRegisterSingletonInstance, then calls to public slots in the base class from QML work fine (e.g. MyApp.myFunc()).

However, if you have a subclass from the above base class, then calls to public slots in this subclass (e.g. MyApp.mySpecialFunc()) from QML no longer work, and you get "Error: Unable to determine callable overload" in the console.

The above worked in Qt 6.4.3 and all previous versions going back to Qt 5.15.2 and maybe earlier.
This error still occurs in Qt 6.5.3 and Qt 6.6.0, so it looks like the other tickets related to this error only partially fixed some things.