I had the same issue and already mentioned it in the wishlist thread, but apparently nobody else has this problem as I got now feedback whatsoever. Using an old KISSlicer version (1.2.0.1) worked for me, but I have not carefully checked whether this was just by accident or whether this version prints the parts always in the same order. I think that printing in the same order is not only helpful for cooling but also optimizes printing quality as all dynamic parameters are kept as similar as possible between layers (e.g., backlash, oozing behavior, etc.). Obviously, not using the same order is also useful at times, that's why I suggested to add another option to the "Multiple object print order" dropdown menu in the "Misc" tab, in the hope that this would affect not only distinct objects but also the islands in the same part (like the "Print islands sequentially w/o optimization" option in Simplyfy3D does).