What happened?
When a message is sent and is then deleted, the mobile app has issues no longer counting it as a message that can be read or unread. As a result, upon opening a channel where the last message in it was deleted, the app will not mark the last message in the channel as read unless the channel is manually marked as read. Going into the channel after manually marking it as read will once again result it the channel displaying the unread message dot until manually marked as read again.