Same happening with me. I have completed around 10 orders, 8 of them with 5 star rating. But there has nothing been changed in the Success Score Stats.
I would like to suggest you, kindly contact with Fiverr support team and provide all the details related to that specific order and team will review and they will definitely help you.
You can report the buyer and also mark as Spam. So, that buyer can not communicate with you, after the new update, there is no any option to block the buyer from our end.