I think it's a bug in RetroArch. RetroArch allows you to choose which favored system be it NTSC or PAL and by default it is auto. Auto I presume means region-free, however the auto option doesn't work. No matter what NES core system you use, auto won't work, suggesting it's a bug in RetroArch not the NES cores. Hopefully somebody more knowledgeable than me could resolve this, otherwise I will have to get all NTSC roms instead.