Official comment
@Ian is correct, the setting is not retained - and the reason for this is it resets when you connect a device. If it were to stay with a particular setting (say, crossfader disabled; no channel assignments), and you then connected another controller which has hardware switches to select the assignment, and it DID NOT change, you'd start to wonder why things weren't working when your hardware settings say one thing but the software says another.
The way to do this is open the mixer portion of the display and deselect each of the channel assignment buttons to either side of the crossfader:
