I’m not a developer, and have no ties in manufacturing.
With that said, I’m about 99% sure this is a hardware limitation. My guess is it was designed so when someone switches to USB, the mic input is completely severed from the signal path.
How it Should Behave is the mic input should run down the USB and at the console input it just takes whatever is sent down USB.
This way, the gain still controls the mic pre, and anything coming down USB is digital anyways so you can trim on the other end.
This is how any Soundcraft, Midas, Behringer, etc. Function.
Huge design flaw and the single and only reason I bring my Si Expression out on gigs 80% of the time now.