There is of course the possibility there are still bugs in the new booking engine.
It took them probably 6-7 months to fix the major issue where once you got to the payment screen that any movements backwards (such as back to the main fare screen) effectively locked out those fares and seats.
On a couple of occasions I booked flights to AU and forgot to select mix fares on one of the sectors (I often book 1 pax with a bag when traveling with my GF). If I'd go back to change the fare type you'd then get the seat select screen telling you the seats you selected were allocated, and then get to the payment screen and find the far class I originally had was now sold out and unavailable. These would both release a few hours later.