Did you cancel the YouTube Premium subscription in the app with Youtube, instead of cancelling it through Apple here:
View or cancel app subscriptions on iPhone - Apple Support
YouTube offers subscriptions both directly with them that they manage and also as an in-app purchase that Apple manages, making it confusing for the user, since each needs to be cancelled differently. How you proceed will depend on the method used to purchase/cancel the subscription. In any case, with Apple taking the money for your subscription, YouTube was paid and you should see the benefits for it. I would also suggest contacting YouTube to see why the subscription is not being honored.