For the entirety of iOS 13 I endured the most annoying Mail.app bug I can remember: The unread mail count, both on the badge and in the app itself, was not being updated unless you force-quit the entire app. Sometimes even that didn’t help. In the above example, I do not have any new mail – easily verified in Mail.app on macOS or the Fastmail web interface. So the list of messages is correct – only showing one read mail, nothing else. It is the count that is displayed inside the app at various places and on the badge that is wrong, still reflecting a mail I read on my Mac a few minutes ago. Refreshing inside the app doesn’t do anything, the incorrect count remains like that for hours. I think it actually only clears once another “change event” is triggered, e.g. actual new mail coming in.
I can’t overstate how disruptive it is not being able to know if you really do have new mail or not from a glance at the homescreen. It is beyond my understanding how something as, frankly, embarrassing could go on not being fixed for an entire release cycle. Apparently not too many people were affected by this particular bug, as none of the iOS 13 point releases addressed the issue.
When I heard about another, equally ridiculous, iOS 13 Mail.app bug finally being fixed in iOS 14, I foolishly hoped for relief. Well, the above screenshots are all from iOS 14.