Had a similar issue on a CDG-LHR flight:
Wife and I travelling with infant managed to get the AC seats shortly after airport control opened.
Seats assigned at check-in were the AC seats, then at boarding all three BPs beeped and red light shows. Turned out they had to accommodate a family of 2 in which the mother was a Gold (heard the CSD say so when taking the seat ).

Wife ended up in CE and I slumped to the back of the aircraft