New mail icon is not showing in the system tray (Windows 11; Thunderbird 159) (bug2087747)

Over the past couple of days, I've discovered new mail in my inbox but the new mail icon (with the red dot) doesn't show in the system tray. I get notifications of new mail, which is fine if I'm sitting at the computer. But the icon in my system tray doesn't change, so I never know if mail has arrived while I was away. I've tried everything - both in Thunderbird settings and Windows system tray settings - and nothing is making a difference. Is this happening to anyone else? Might it be a bug? Averill