Mainstage not muting external instruments

I am running an external keyboard into MainStage, but when I have a patch with no external instrument channel strips on it, there is still audio signal coming from the external keyboard.

I have made sure there is no monitoring going on with my interface, and the only solution I can seem to find is by soloing the MIDI channel strips I wish to use in order to avoid mixing them with an unwanted external instrument sound.

Has anyone ever had this problem? I have been using MainStage for years and have never run into this. Currently running on Mojave.