Update: The website is still doing it to me.

Rang Iberia+ Oro line and they also quoted me the a figure in the 26,500 range for a mixed Azul+Turista MAD/LHR/MAD, which I questioned pointing to their own redemption charts showing the lower figures (Azul+Turista should cost 16,250 off-peak for this particular routing). I pointed out that 26,500 Avios would put MAD/LHR/MAD in the Band 3-4 range, which makes no sense. The customer service agent was also confused after looking at that. After a very long wait (half an hour on hold! Good thing I rang the free line…) she came back to confirm the fare was wrong and there are gremlins in the system (it's Iberia, what's new there), and I should be looking at the 16,250 amount.

Seems like, as usual for IB, there are IT problems – so much so even the internal systems are showing the wrong Avios redemption fare. She said she was told that for some customers, they don't know why, the system pulls a wrong table. She was able to reserve my ticket for me paying just the taxes, with the Avios being charged manually separately from their end instead of through the system.

If you call their call centre back, point them to the redemption charts and see if they can do anything for you to get you the right fare. (Their redemption charts are on their websites under Iberia Plus partners > Flights > Iberia.)