There is a bug where the iOS badge number does not clear (or decrease) when the mention is in the last channel that was viewed and the user opens the app with the icon instead of the push notification.
Repoed on build 168 5.7-RC4. (This bug repos on 1.15 as well but not 1.14)
Make sure the icon has no badges showing
Open the app on iOS
Note the channel you are currently viewing (repos with all type. Channel, GM and DM)
Place the app in the background
As another user, mention the test user in the channel noted above
As the test user receive the notification on your phone and note the badge now says "1" on the MM icon
Select the icon to bring the app to the foreground
The post with mention is visible in the channel
OS notification and badge on channel drawer have cleared but badge with "1" remains on icon.
Does not repo if opening the app via push notification.
Interestingly, when you receive the next notification, the badge count does not increment so if you don't hit this bug again all badges will be cleared.
If the badge is cleared form another client it is cleared from iOS icon
If the app is in the foreground and your viewing a different channel, the badge is cleared properly when you open the channel
If the app is in the foreground and your viewing the channel the badge is cleared properly