not sure that's entirely true Charlie. the OP says the problem only occurs with a key change from A to B. if i understand Andrew correctly what he's saying is BIAB generates the whole song as if it were still in the key of A. If the OP starts the song in B the RealTrack works fine. the problem wouldn't exist if BIAB was 'able to follow key changes within the song for RT generation' as Andrew puts it.

the recorded material is fine - no need for extra recordings - just BIAB can't always cope with key changes for RT generation.