A JWT is not encrypted but signed. The payload, eyJkYXRhIjoiSGVsbG8ifQ, is just in base64. The signature probably doesn’t match.
It looks like GPT-4 returned a very common JSON {"data":"hello"} in base64 with some random and wrong signature.
Did you ask for a JWT before in the context ? It’s weird that it didn’t provide a JWE document.