...your comment made me think about deleting the notes displayed in the RealTrack's notation window, i.e., the MIDI notes. Well, after deleting the displayed notes, I no longer hear the piano at the same time as the guitar!
This sound like an inconsistency (and maybe a bug). My take on it is that the underlying MIDI notation for a Realtrack (generated on a Utility track) does not have its volume output either disabled or set to zero during the generation process.