The Galaxy S8+ is a great device! Let's take a closer look at your messages. May I ask if you are seeing any type of notification on the main screen? Is this just occurring with your texts messages?
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On the s7 edge its in the lock screen and security -notifications on lock screen then see if messages is in there I have mine on and it always shows me the message when my phone has a dark screen. It should be about the same on the s8