I have had that problem with small diameter trees where I spilt than in half, but the more splits I get out of them, I seem to have less problems. Halves are the worst, quarters less so of a problem, and we'll see on these sixths. Trees that are leaning are a whole other story, them seem to twist and warp much worse even when splitting perpendicular to their lean. With the slight warping I have got with the halves, it was nothing that wasn't fixable with a little heat treating.
Eric