I booked DFW -MAD (Aug 26) using BA avios miles on IB. Got the same message. I opened up IB Club account, went in 1x week to try and add seats. After many, many fails it finally worked and the seat assignments stuck. I can't explain it. Fingers crossed my seats do not mysteriously vanish. Give it a try and good luck to you.
btw- we have NO status with IB AA or BA