I'm not able to reproduce this consistently, but often when I click on the last remaining unread notification in my inbox, it goes to the question but doesn't remove the count badge. So there's a "1" badge, but when I open the inbox there are no unread notifications. There's also no "Refresh", so it's not a case that new notifications have arrived but aren't shown yet.
4
badge until I clicked "Mark all as read". Unlike the original problem I posted above, it stayed even when I refreshed or switched pages. I haven't seen a repeat of it yet.