It looks like I finally found the "trick" to avoid the problem. PP2012 in the rendering settings comes out with a "Min shading rate" of 0.95; putting that value to 0 is all that's needed to avoid having seam lines in rendering. It's a problem occurring only on a small amount of V4 maps, so it's not very likely to trouble a lot of people, but, to my knowledge, it's easily corrected just putting the Min shading rate cursor to 0. I hope someone can find it useful.