To expand on this, I'm flying in just under 72 hours from now in Y and took a screen grab of the seat map just before the 72 hour point. Looking again shortly after 72 hours showed a big difference so I'm assuming the auto-assignment has just happened. No extra Preferred or Twin seats were allocated, but starting at Row 25 there were lots more filled, however still several gaps, almost all single seats. I would suggest the gaps indicate that parties are being sat together and single seats on their own are being left. It also seems they work from A to K as all the A window seats (except for one single on its own) are taken and no additional K seats taken.