I found when slicing a completely round object with no infill and 2 perimeters 1.2.1 is giving strange results while 0.9.9 is working well. See the picture.
It may well be an issue with the STL that different slic3r versions react to in different ways. An STL file cannot actually represent anything except straight lines, so a circle is always modelled as a polygon. Try running the STL through a NetFabb repair.
