QSystemTrayIcon balloon message not showing on El Capitan - c++

Question

I want to display balloon messages with QSystemTrayIcon in my qt app on mac.
The tray icon appears in the right corner of the menu bar of mac. The problem is the balloon message won't show on El Capitan (10.11.6), but it does on Sierra (10.12) and High Sierra (10.13).
The Qt Tray Icon example works just fine on El Capitan though.
QSystemTrayIcon::isSystemTrayAvailable() and QSystemTrayIcon::supportsMessages() both return true.
I don't know what to do anymore. Here's my code:
custom tray icon class (.h)
class MyTrayIcon
{
public:
MyTrayIcon(QObject* parent = nullptr);
~ MyTrayIcon();
void balloonMsg(const QString& title, const QString& message);
QSystemTrayIcon* sysTrayIcon = nullptr;
};
custom tray icon class (.cpp)
MyTrayIcon::MyTrayIcon(QObject* parent)
{
sysTrayIcon = new QSystemTrayIcon(QIcon("my_icon.png"), parent);
}
MyTrayIcon::~MyTrayIcon()
{
delete sysTrayIcon;
}
void MyTrayIcon::balloonMsg(const QString& title, const QString& message)
{
sysTrayIcon->show(); // the icon does show
sysTrayIcon->showMessage(title, message); // the message doesn't show
}
Main window class (.h)
class MainWindow : public QMainWindow
{
Q_OBJECT
public:
explicit MainWindow(QWidget *parent = 0);
~MainWindow();
Ui::MainWindow *ui;
MyTrayIcon* mTrayIcon = nullptr;
public slots:
void btnClicked();
}
Main window class (.cpp)
MainWindow::MainWindow() : QMainWindow(parent), ui(new Ui::MainWindow)
{
ui->setupUi(this);
mTrayIcon = new MyTrayIcon(this);
connect(ui->pushButton, SIGNAL(clicked(bool)), this, SLOT(btnClicked()));
}
void MainWindow::btnClicked()
{
mTrayIcon->balloonMsg("my title", "my message");
}
Thanks in advance !