I had the same problem on my 55 after 18k miles, It has taken be 2 months to get Nissan to agree to fix it.
Had the seat back replaced last Friday! Only problem is the new one is a different colour and different type of leather.
Dealer has told me that the part number for original seat has been superseded, therefore there is nothing they can do and told me to complain to Nissan customer services asking for both seats to be replaced now if they can't get the part to match.
Waiting to hear back from them