I’ve flown this recently, though I booked via BA on codeshare. The BA call centre was able to reserve a seat in the PE section for me, not sure they will do that on an IB-issued ticket.

Generally when booking IB flights via IB (with a BAEC number in the booking, though not sure how much difference it makes), I’ve had more success with seat selection using the IB app rather than the website. I realise this doesn’t answer your specific question, but maybe worth a try.

It was a pleasant flight, apart from the remote stand and bus transfer for boarding at MAD.