This happened to me on flight to Frankfurt a couple of months ago. Was in Y and had booked exit row for me and my partner - when it came to online check-in, he had been moved to bulkhead middle and I was on exit row window. Called the hotline and they said it was a computer generated move???? I complained and complained and we got two bulkhead seats next to each other finally (flight was full and they wanted us to sit separately) both of us are CX gold.