reply from notification

As part of new feature on 7.0, I noticed when I reply from notification for hangouts message, after I enter the reply and hit send, it sends the message and the notification closes and goes away. However I noticed if I do the same thing on Google messenger, when I reply and it send, it adds my reply to the notification and the notification remains until I clear it. Anyone seen this? Could it be also app specific behavior, or maybe a bug? Just curious.