This happens to me as well. I've found the workaround is to always make hit "home" after finishing with the app before turning off the screen again. I've tested

This happens to me as well. I've found the workaround is to always make hit "home" after finishing with the app before turning off the screen again. I've tested it, and if I leave hangouts as the on screen app when I turn off the screen it doesn't notify me of new messages.