After the latest Oreo update on my Samsung galaxy S9+, with Sprint, I can no longer use my individual text notifications for each contact. I am using Go SMS pro not Samsung's stock messaging app. When I check the go sms app my ringtones are there and assigned and I can play them, but when I get a text it just plays the tone set for general notifications on the phone. Up until yesterday it worked fine. Then my phone updated.
Have you checked to see whether an update is available for the app itself? Alternatively, you could try uninstalling the app and reinstalling, to see if this makes any difference.
Let us know how you get on.