I do not see that this has been posted before, but I think its rather weird.

When I use the Linux Mail app and I get a new mail, it changes the badge for number of mails. However, I mark the mail as read, it stays there until I restart the app. Thats quite annoying, I always have a 1 or 2 there when I read new mails.

Something similar happens to the iOS-app. If I read something in the linux app, the counter on the iOS app is not going down, until I open the app.