I have one of these but 3 out of 4 segments are in business and one was booked in Y class.

JFK - LHR: IB flight number operated by BA - I class
LHR - MUC: BA flight number operated by BA - Y class
MUC - MAD: IB flight number operated by IB - I class
MAD - JFK: AA flight number operated by IB - I class

Trip ticked by AA.

Couldn't book seats at all without a fee until i discovered when working with the itinerary using the PNR that my AA FF number didn't transfer. Once I entered that, I was able to secure seats on 3 of the legs (all except the MUC-MAD flight).

Waiting to see what will go wrong with this one...............