I experienced similar a few days back on similar Aer Lingus flights, two issues actually - AVIOS showing 5 seats wouldn't allow me to dummy book via AVIOS 3 adults and 2 children, but would allow for 5 adults to be booked, and secondly BAEC couldn't see the AVIOS availability that I could see via AVIOS.
I slept on it and the mismatch appeared to resolve itself by the next morning.