When tickets are cancelled, the space goes back into inventory. Not only is there no guarantee that the space will become available immediately or ever again in a redemption bucket, but there is no guarantee that the space will ever be sold. Could be that BA's RM/IM algorithm is predicting an overbooking turning into an oversale. In order to avoid the latter, BA won't release the seats yet or at all and thus the flight will show as sold out even though you have just returned seats to inventory.