Automix assumes crossfader is set to "default" mode.

If the crossfader is set to “cut” (or other modes) mode the fade doesn’t work as expected (in “cut” mode there is no fade at all). The application should switch the crossfader to the auto mix algorithm’s assumed “normal” mode when in automating mode or use the track’s volume faders instead of the crossfader to do the fades, disallow fades, or some other solution other than just not working properly.