If I inherit a class from QMessageBox I don't receive the done() signal when a button is pressed. The code is built using Qt5.14.2. However with Qt5.12.0 it is called.

class QtQuery : public QMessageBox
{
_Q_OBJECT_
public:

QtQuery(int a_button1,
               int a_button2,
               int a_button3);

//! Destructor.
    virtual ~QtQuery( );

protected slots:
     void done(int a_val);
};

 ...

void QtQuery::done(int a_val)
{
std::cout << "Done is called - result: [" << a_val << "]" << std::endl;
}

Launching the attached example built using Qt5.12.0. Just push any button to close the message box:

Testing ....
Done is called - result: [1024]
Testing ends with ret: 1024

Launching the example built using Qt5.14.2:

Testing ....
Testing ends with ret: 1024