GarageBand strings always play violin when MIDI controller is used

Dear community,

I am connecting an external MIDI keyboard to my iPhone. When I add a string track, no matter what part of the ensemble (violin, viola, cello, or bass) I select, the notes played on the MIDI controller are always played on the violin. The touch screen instrument does not suffer from this, so if I select cello, the controller plays the violin and the touch screen plays the cello.

My obvious question is: What must I do in order for the part selection to affect not only the touch screen instrument but also the MIDI events?