Parameterizing a QML component with a registered C++ class having a QProperty

[bootchk]

I have defined in Python (alternatively C++) a class A which has a notifiable Qt property B. I registered the class A with QML. I want to parameterize a QML Component with the property B of class A.

This doesn't work, it quietly fails to emit a signal 'changed' from property B.

main.qml

@import MyComponent 1.0
import A 1.0 // C++ class A is registered

Item {
model: A{}
MyComponent.MyComponent{ componentModel: model.b } // specialize component with model: property b of instance of class A
}@

MyComponent.qml

@MyComponent {
property var componentModel
...
onActivated { componentModel = 1 }
}@

This works: same as above except:

@ MyComponent.MyComponent{ componentModel: model } // specialize with class A@

and

@ onActivated { componentModel.b = 1 } // component accesses property b of instance of A@

Why doesn't the first example work? That is, why can't I pass a QProperty to parameterize a component?

There is a discussion here: http://doc.qt.io/qt-5/qtqml-cppintegration-exposecppattributes.html, but I think that I have properly exposed (to QML engine) the class A and its property B.