i have had this issue and it is clearly the BA IT yet again. i fly frequently LHR-MAD-BIO and have been caught out going through the BA site booking and paying for biz to see after purchase the IB leg was booked into econ, but i caught it near immediately so was able to ring up and have the agent book the flight into all biz for the same fare.

it is annoying because i try to avoid booking with IB directly but have to call BA to make these bookings because clearly something on the IT is not being fixed. and this has only happened with BA-IB mixed itins