This is more of an opportunity than a problem at this point. How many errors do we make in shooting where we have no idea what the error is; the arrow just misses the target? This time, you know every time you make the error, because your fingertips sting, and every time you don't, because they don't sting.
The error is not relaxing your fingers when you release the arrow. This is different from creeping, although both can happen together.
If you are conscious of your release, it is much more likely to happen than if you aren't. A grip sear is supposed to act as a non-anticipatory psycho-trigger, but you are probably anticipating it anyway. Don't think about the release and just let it go off by itself.
Rod Jenkins likes the bucket drill, where you hold a bucket full of water at your side with your usual string grip, and learn to let go of it without spilling any water, which will happen if you relax your fingers, but not if you consciously try to open them.