Unless one has some status, advanced seat selection at QR for Y redemption booking should be chargeable.
However, I do notice the system is very buggy. I used some points to redeem seats for one of my bookings, the initial payment was ok and an EMD was issued to associate with it. Later on when I choose to change to the seat next to me, the system will be showing as confirmed BUT the EMD associated will be lost. Each time I had to call and get it re-associated, and I was even forced once to pay extra to cover the differences for the same seat type as they changed the pricing overall (I paid extra to move to extra legroom seat instead)

I remember that for first-time seat selection, if you click some buttons to exit the booking before payment stage, the seating would actually be saved in the booking without EMD issued. My guess this is the situation OP was in.