Iberia is not natively on Amadeus (even though they own or used to be part owners of Amadeus) and seating assignments between BA to IB codeshares is really rocky. You could spend your life trying to manage this and still get a last minute change. So your best bet is when you are near to departure to monitor this using the BA PNR on the Iberia app (not website version). It's just one of the downsides of codeshares.