I have encountered problems with IB bookings seat selections on the MAD-LHR route with IB operated flights with seatmaps under random BA/IB control on the websites, but this is a new one:

Just made a booking on Iberia Express OVD-LHR-OVD. Ignored offered to purchase seat selection (as OW sapphire have this privilege for free) and proceeded to complete booking. Strangely, in Manage my Booking on IB's site, option to select seat not shown.

When accessing the booking on BA's site, when you choose to access seat selection it diverts you to IB's site for the outbound sector (OVD-LHR) where you can select the seat. However once you do that a dialog box comes up saying there is an error.

The inbound segment appears to be operated by BA. Here you are allowed to choose seats on BA's seatmap immediately in front of and behind the exit row, but no others.

A BA telephone agent confirmed that both flights are actually operated by IB express and was able to confirm the desired seats for me.

I don't know what's going on with IB seating recently. I have an IB Express flight marketed as IB on a ticket issued by CX and it just says to call. There have been lots of reports recently of weirdness as well.

Usually you can actually get decent seats by OLCI. It seems it's only working for directly booked flights lately.

I've never managed to get seat selection a non-IB issued ticket even if the flight is IB-prime coded.
I just use rj.com - never had an issue, but it doesn't show which seats are already occupied.

BA.com has an issue in which exit rows are not being selectable for many people.

I've been having all sorts of issues with IB seat selection (for the past few months). I eventually got seats selected for my AA ticketed flight, but now have no option to change them. The call center wants to charge me seat fees even though I have OW Sapphire .

For my IB ticketed flight, on the other hand, I was able to select and change them, and am still given that option.