I think that's one of the drawbacks to the system. When you leave it alone it will shuffle the songs, but if you use the forward button it just goes to the next one in the list. I haven't figured that one out either.
definitely something that they should fix.. but that is true, if you put it on MIX and leave it alone it works on mine, but as soon as you press |< or >|, MIX is turned off -- possibly the dumbest thing i have ever seen..
even if you switch to FM, then back to SD, MIX is off.. at least it stays on MIX when you go to NAV..