Can you confirm which model phone do you have? What software version did you update to? You can find this under Settings > About Phone > Software Information.
The new update was pushed in the text app, however you can only assign a text tone to an existing text/thread. Once you delete the convo the customizations is gone and reverts back to the default text when that person text you again....this is the most ridiculous spot to fix this issue unless the customization doesn't get deleted which reduces the frustration by 50%..
It needs to be a field in the contacts list like it was prior to the Oreo update. Where you can preset a text tone regardless if you ever get a text from that contact. I'm forced to keep my text convos if I want to keep the customize tone for each individual, or forced to get a 3rd party app.. It may be nice to have the latest and greatest phone but if simple features that make a phone a phone such as "hearing" who's calling and texting prior to picking up the phone are removed then it's not worth upgrading.