​​​Unless it was a change of aircraft messing things up, then I’m guessing that’s the recent iOS update to blame (if indeed you checked-in via the app): it doesn’t seem to remember the previously chosen seat and (adding to misery), once you’ve flown the first leg, it will no longer show the return flight or indeed any subsequent segments. It is clearly a bug which I’m hoping they’ve sorted with the fix/further update a couple of days ago.

G