I have had this since 10.0 beta 1 on my 6S plus. Tried doing a clean restore with 10.0 final and also set my 7 up as new when I got it. In both cases, this bug came back after a while as more text messages built up. I think it might be related to points in the conversation where an iMessage failed to send and had to be sent as a text message. But, I'm not sure. It's just one in the long list of unresolved iOS 10 bugs. I usually just reboot the phone whenever it happens.