This led to some unusual behaviour in the UI as the property was used in some logic to determine whether to show a particular entry in a popup menu. Although I have fixed it by re-exposing the property, I am baffled that there was not more of a run-time failure from QML. The code is something like this:

    ...
    if (data.isCopyEnabled)  // <<<< isCopyEnabled not exposed from gadget
       ... // add "Copy" entry to menu
    if (data.isDeleteEnabled)
       ... // add "Delete" entry to menu
    ...
    menu.open();
}

I have things set up so that I see QML runtime warnings in my console, and I see nothing there when this executes. Also, it is clear that execution continues over the data.isCopyEnabled check because the other entry is added and the popup is shown. The behaviour is as if isCopyEnabled is false.

Is this expected behaviour? I am sure I have seen more noisy behaviour of one type or another in the past when I have attempted to access an attribute that does not exist on a C++ object exposed to QML.