//main.qml
Window {
    id: window_main
    visible: false

    LoginWindow {
        id: window_login
        visible: true

        onSuccess: { // не срабатывает
            window_login.visible = false;
            window_main.visible = true;
        }
    }
}

//LoginWindow.qml
Window {
    id: window_login
    visible: true

    signal success()
    ...
    Button {
        ...
        onClicked: {
            ...
                    if(connected) {
                        //Succeeded
                        window_login.success();
                        console.log("LOGIN: OK"); //вызывается
                    } else {
                        label_errorMessage.text = loginDB.lastError;
                    }
            ...
        }
    }
    ...
}