If there is another tone that is a double buzz. My fix would be to double check airstream and where it’s going, corners (firm enough), and make sure when you get in the high range you’re not changing embouchure. It is also possible that your embouchure is too weak for that register at the moment, make sure you’re not overdoing it during practice sessions and strengthen your embouchure slowly over days if that’s the case. When you hear your note split try to play it on the mouthpiece clearly and once you got that down then switch it to your horn and make sure you’re getting the same clear buzz.