TrayIcon shows on one mac but not on another

Question

I've knocked together a simple PyQT5 app that displays a trayicon.

On my mac it works:

However on my client's mac it doesn't display. (Alas, I do not have a screenshot).

I noticed my client has more items on his tray, so my first thought was that there's a limit, and he had hit that limit.

However, removing one icon and re-running, still it does not show up on his system. Which would appear to debunk this hypothesis.

Can anyone think why it's not working?

Here's the code:

import logging
import signal
import sys
import subprocess
import os

from PyQt5 import QtGui, QtWidgets, QtCore

logger = logging.getLogger(__name__)


# This is't used...
def refresh(*args):
    logger.warning('Hello')


class Application(QtWidgets.QApplication):
    def event(self, e):
        return QtWidgets.QApplication.event(self, e)


class Application(QtWidgets.QApplication):
    def launch_terminal(self):
        # start backup script
        cmd = f"cd {os.getcwd()}; env -i PATH=$PATH bash -c ./backup.sh"
        subprocess.run(['osascript', '-e', f'tell application "Terminal" to do script "{cmd}"'])

        def check_backup():
            if os.path.exists('/tmp/backup_done'):
                os.remove('/tmp/backup_done')
                self.quit()

        # start timer to check if backup is completed
        self.timer = QtCore.QTimer()
        self.timer.timeout.connect(check_backup)
        self.timer.start(1000)  # check every second


class SystemTrayIcon(QtWidgets.QSystemTrayIcon):
    def __init__(self, parent=None):
        super().__init__(parent)
        self.menu = QtWidgets.QMenu(parent)
        self.refresh_action = self.menu.addAction("Refresh")
        self.refresh_action.triggered.connect(refresh)

        self.exit_action = self.menu.addAction("Exit")
        self.exit_action.triggered.connect(QtWidgets.qApp.quit)

        self.setContextMenu(self.menu)

        # Create a pixmap from a Unicode character
        font = QtGui.QFont("Arial", 30)
        pixmap = QtGui.QPixmap(50, 50)
        pixmap.fill(QtCore.Qt.transparent)
        painter = QtGui.QPainter(pixmap)
        painter.setFont(font)
        painter.drawText(pixmap.rect(), QtCore.Qt.AlignCenter, "")
        painter.end()

        self.setIcon(QtGui.QIcon(pixmap))


def main():
    app = Application(sys.argv)

    # Registers a signal handler to exit the application gracefully
    # when receiving a SIGINT signal (e.g., Ctrl+C on the terminal). 
    signal.signal(signal.SIGINT, lambda *a: app.quit())

    tray_icon = SystemTrayIcon()
    tray_icon.exit_action.triggered.connect(app.quit)

    tray_icon.show()

    # Timer calls Application.event repeatedly.
    app.startTimer(200)

    app.launch_terminal()

    sys.exit(app.exec_())


if __name__ == '__main__':
    main()