An unread message notification that won't clear can be removed by marking all your messages as read from the Messages tab.
Why this happens
The unread message notification usually appears when another user has sent you a message that includes a picture or emoji. The system sometimes has a hard time registering that you've seen the message.
How to mark all messages as read
- Open the Messages tab
- Tap the three dots in the top-right corner
- Tap Mark all as read