Can you post the gcode somewhere? Maybe there's some kind of configuration issue with the purge lengths or something. During the initial purge moves it shouldn't move the extruder backwards AFAIK. I haven tried Prusa Slicers multi-material support for a while so I can say it's not meant to do that, though... I'll see if I can get Prusa Slicer configured for my 6-color machine, at least try so I can see how it does things.