Thank you for the suggestion.

I think, I find the issue.

Setting a parent to QMenu fixed the issue.

menu = QMenu(parent=toolbar) or
menu = QMenu(parent=self).

Your suggestion - setting the menu as instance variable , also works fine for me.

Thanks again.

Thanks for checking.

I observer this this issue on Mac, I am not sure if the behavior is different on other platforms.

Can you check the same with:

import sys

from qtpy.QtWidgets import (QApplication, QAction, QMenu, QWidget,
                            QVBoxLayout, QToolBar)


class Widget(QWidget):
    def __init__(self, parent=None):
        super().__init__(parent=parent)
        layout = QVBoxLayout(self)
        toolbar = QToolBar(parent=self)

        self.menu = QMenu()
        self.menu.addAction('one')
        self.menu.addAction('two')
        self.menu.addAction('three')

        self.action = QAction( "Number", self)
        self.action.setMenu(self.menu)
        toolbar.addAction(self.action)

        layout.addWidget(toolbar)


if __name__ == '__main__':
    app = QApplication(sys.argv)
    icons.init()
    wid = Widget()
    wid.show()
    sys.exit(app.exec())

int main(int argc, char* argv[])
{
    QApplication app(argc, argv);
    QWidget w;
    auto layout = new QVBoxLayout(&w);
    auto tb = new QToolBar(&w);
    auto menu = new QMenu(&w);
    menu->addAction("one");
    menu->addAction("two");
    menu->addAction("three");
    auto action = new QAction("Number");
    action->setMenu(menu);
    tb->addAction(action);
    layout->addWidget(tb);
    w.show();
    return app.exec();
}