Well,
I dont use garage band software so my answer will be generic…
What you post says is garage band is summing all the tracks to a stereo pair…
This is normal behavior for most basic daw systems as your end result will be a stereo pair output to cd most likely, and the all believe all mixing should be done in thier software as its the best or so they think…
What you need to do is look at the routing of the tracks…it’s a software mixer…
Ex. Track one will show output to master
This means the track is routed to the master fader (track)
What you want is to mute the stereo pair (master tracks)
Then rout each track instead of to master to the return channel on the mixing board…then on that channel on the board you need to select USB for input…
Garage band may use different terms, but hopefully this will get you going…
Btw there are some very nice daw’s cheap or free out there…if garage band is giving you issues…