I created my own JABB template before I was aware of John Barron’s, but both templates are exhibiting the same problem. Within the trumpet and trombone sections, the mixer only shows level on trumpet 1 and trombone 1, regardless of which player in the section actually is playing. In addition, the faders for chairs 2-4 in both sections have no effect.
To reproduce the issue, create a new Big Band score and replace the playback template with John’s JABB template. If you enter notes in trumpet 2-4, you will see level only on trumpet 1 when you start playback.
I feel like this is an Aria issue, but I have yet to find any relevant controls to tweak, nor other users on the internet having the same problem.