This happens to me as well sometimes. Or if I have a thread open and receive a message. I don't have to reboot. I just simply close the app in the multi-task switcher and open it again and it fixes itself.
Where I mostly get it is when I receive another message from another thread while one thread is open. If I go to the new thread where the new message is supposed to be, it doesn't show until I completely close the app and open again. Same instance only when sending a message, too.