Hello! Let's try this, open the Contacts app. Find the contact you wish to modify. Click the small circled "Details" button. At the top of the next screen, click the Edit button. In the edit screen, scroll down near the bottom, right above the scan business card button, and click on the View More option. Scroll down and you will find the text notifications option.
this is something that is getting looked into since it popped up after the Oreo update. this is affecting S8's across multiple carriers.