Hi Jeffrey,
Unfortunately, that seems to be a limitation in GarageBand.

I just tried it here to confirm that, but it seems that only the plugin windows for the selected tracks get displayed.
As soon as you select the audio track for your guitar input the instrument track plugin disappears.

Do you have any other AU hosts to try ?
I know this is not a problem in Logic anyway,

Alternatively, I know it's not ideal, but you could try to record guitar track into the audio track in the BIAB main application.
Sorry wish I had a better answer.

- Jay

MacPro 5,1/12 core@3.46GHz -- OS X 10.15.7 -- ATI 7970 -- 32 Gig RAM -- Crucial 500GB OS SSD -- Samsung EVO 1TB Audio/Sample SSD -- BIAB 2024 -- Logic Pro X -- Cubase Elements 14 -- Too many plugins