I know its been 4 years, but since the subject is still an issue, I would like to add my comment here: In my opinion, this is most certainly a bug and should be fixed!
I don’t quite understand why a record enabled GA instrument track would accept midi information from a chord track, event though Midi Input on the GA track is set to “not connected”. Why does record enabling a track override Midi “not connected”. It does not make sense to me, sorry! If there is no input bus selected on an audio track, record enabling the track also doesn’t play back audio from other tracks. So why does this happen on my Groove Agent instrument track?
Aside from that, why should a chord track ever trigger a rhythmic engine like Groove Agent?? Anyone need chord drums?
I spend hours troubleshooting the issue, until I finally stumbled upon this thread (thank you shadowfax) and realized this is an unfortunate configuration issue slash borderline bug.