The host sized the grid from 2*max_hkl while the kernel guards against 2*max_hkl+1, so whenever the rounded-up grid landed exactly on 2*max_hkl threads (max_hkl a multiple of 4, with the 8x8x8 block) the h = +max_hkl plane was never launched. The CPU loop runs -max_hkl..+max_hkl inclusive, so the GPU predicted a strict subset. Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>