Hi, i’m also having this problem.
3.1.7 seem to work fine with 2011 theme but with my ‘elegance’ theme, when I import a category i.e. belts using ‘Accessories>Belts’in the csv, they are importing into Accessories>Belts and Books. These categories appear as ticks in the category boxes in the back end and also as actual products in the front end. This also happens with other products importing into their correct categories and another random one.
Rolling back to importer 3.1.6 solves that problem.