Oh, that's too bad. IIRC, the official line from Kawai has always been that audio should be recorded from the line-out (or USB in this case) for the best fidelity. And I get it, it takes out the far more variable effects of microphones, environment, recording experience/skill etc.
But then how are you supposed to capture/share the quality of the speakers/soundboard/amplification? Especially for something like the NV12 where the sound output is really supposed to be flagship quality.