This is neither a bug nor any problem. The response rate and rating evaluation of fiverr during last 60 days in period. You have to understand it clearly.
About response rate : Your previous average response rate calculated 91% within last 60 days of that time replayed/not replayed to number buyer's first message. After some days number of replied message decreased within last 60 days and average response rate become 89%. It may decrease more if not replay to any new message. But all will be reset after 60 days ( from the day of not replayed message) and response rate will be 100% if you replay all upcoming messages.
About rating : Your last review is 4 star within last 60 days. So it is showing 4 star. If you get new order and get 5 star, then positive rating will back 5 star. But if you not get any order / not get any review, after the period of 60 days from the date of last review positive rating will become ( N/A ).