Firstly OP if in economy unless there's a reason you can't, always chose exit rows, they're just a little better. Sitting behind the curtain is a risk as you found out. Mostly the exit rows will survive the curtain move I believe, although there's no guarantee of that.
But like so many things with BA, and maybe other airlines too, when it comes to seat selection there's so much you need to be aware of that they don't cover on the website. First and foremost is that BA care nothing about you, you're meaningless to them, if they can sell your seat as a CE one, they'll shift you with no regard that you chose that seat based on your status, and they are not going to go to the effort of a more complex rearrangement, they'll just shift you to suit themselves, it sucks, but it's reality. They can describe this sort of thing as operational reasons all they want, but like many things described as operational, they mean economic.