Yeah, the seat of Azem has existed just as long as the others, it seems you misunderstood what Hythlodaeus meant when he said the Convocation was only 13 members right before the Final Days reached Amaurot, it was because the unsundered WoL (the most recent Azem) had left the Convocation, 14 was always the standard number.

And the second round of unsundered sacrifices (in order to rejuvenate the life of the planet) still occurred too.