I agree this is more of a problem that Iberia doesn’t use Amadeus and especially the code share flights are the problematic with a BA 125 ticket number.
If you book everything on IB then the website works ok ish except when it doesn’t.
Usually I have bookings where some flights operated by BA some with IB with mix of code share and native flight numbers. BA pnr usually works on IB website to display all the flights but you can’t change anything. The IB pnr mostly doesn’t work on the IB website or just displaying partially the bookings and you can’t change anything. RJ website just works with the observation I have above in regards of seatmap.
BA contact center almost never able to reserve seats on IB flights.