That is what you should check your updates for messages in. You may have already had it and havent realised or its a steady roll out, I'm not sure. But the rest of my post should point you in the direction if the update is on your phone already.
This is the latest version I have. If yours is the same you should be abke to asign notification tones to each thread.
There is an another option if you dont have update for app yet. Download and install apk manually from Apkmirror. Just download the newest one and give it a try. It works for me on S7/S7 Edge, S8/S8+ and also S9/S9+. Let me know if it helps. Regards.
Edit: ON S8 220.127.116.11 works aswell.