Saw a review from another page,
"For me, it happens when I exit the messages app with a conversation/thread still open. I think when you exit the app, iOS doesn’t recognize it and it’s as if you still have the app on the screen. Thus, no notifications and the ‘read’ status on the other person’s side.
Until a fix, I just make sure to exit a thread before exiting the app."
I have the same issue with multiple message apps, such as WeChat, Messengers, Whatsapp, Instagram, Gmail, and so on. I personally don't use iMessage. I contacted Apple and the specialist said that it is the app's issue, not Apple's. It doesn't make any sense if it happens to multiple apps.
Anyone with a solution please hit me up!!!!