This too has happened to me in well over half my recent UK/European flights when booked in Y. As Gold I've selected an emergency exit aisle seat, but because of C/Y configuration changes and/or aircraft changes, I've been sent down to the back, usually the last row or second last row, but at least I've always maintained an aisle seat!

Whenever this has happened I've moved as far forward as I could, however I think that in just about every case I later managed to get back to an emergency exit, aisle seat.

Exactly why the algorithm sends you to the back I've no idea - there must be some (il)logic to it!