This is an odd one but I suspect the aircraft at the time of booking was a 320 and it was subsequently changed to a 321 and this is what caused the problem. On expert flyer, I have noticed many times that if it’s an ‘aircraft change’ from a 320 to a 320 (which basically means it’s the same frame but the curtain has moved), then our 1a/1c seats remain. However, if it’s from a 320 to a 321, we are booted out of these seats with the same message as you and we’re not reallocated, even though there are no seat number changes on the left of the aircraft, only on the right with the addition of 1d/1f, and we’re both Gold. Other than BA’s somewhat questionable IT, I have no idea why this is the case but I’d suggest setting alerts on expert flyer so you can be notified of any changes (curtain or frame and often before you get the email from BA) and so you have first dibs on your preferred seats and can call in if there’s a price difference. Have a lovely break.