Short answer: at some point the plane has changed or the CE curtain swallowed up the exit row in which case all seat reservations are wiped even if the curtain moves forward again. It's not that uncommon in the fortnight before a flight but if it's any consolation being Gold wouldn't have offered any more protection!

Could BA tweak their systems to auto reallocate previous seats? Possibly. Should they at least send an e-mail telling you your seat has changed? Definitely. It is what it is though & fortunately a relatively short flight