There are probably two issues here. There is a bug which regular A320 users will know that sometimes gives "middle" to seats which are in fact aisle or window. Just a data glitch, been around for a few years, and the map seat plan will be clear as to whether you have an aisle seat or whatever. Separately playing curtain roulette at BA - and you can search for the that term in this forum - tends to result in being cast back to 29E when the seats get programmed into FLY. So you need to be careful about this, not least because 29E may be the best available seat in the house at that stage of the game. And sometimes you can't move out of the new CE row either.