Spoke breakage at the thread end is not really related to spoke length. After all, all spokes, like all screws have some unused thread in tension below the nut.

Usually when spokes break at the nipple it's because the nipple doesn't line up well with the line of the spoke causing a bend where the spoke emerges. If the last thread is just inside the nipple so much the worse because the bend will be at a thread rather than the full diameter area. Look at the rest of the wheel and see how the nipples and spokes are aligned.