I flew LHR to MAD on the A350 on Saturday from terminal 3. Got to the gate and on expertflyer 5L ended up showing as free after check in closed. I went to a guy at the gate and asked if I could move. He told me that the plane was overbooked and there were no spare seats. I showed him the expertflyer seat map and he said it was wrong. As I boarded I was in his line and I asked him again and he was clearly annoyed and said “I’m not lying to you”. He never checked the computer. Got on board and the door closes. 5L is free, but obviously also being eyed up by someone else who jumped into it from their seat. Quite annoying. Had an American Airlines badge and didn’t see any Iberia ground staff. Is it all just communal staff?