@j.p.s: Ah, we may be hitting another fun specificity of my test image:
It grayscales fairly specifically differently depending on whether that was done linearly, or gamma-correct.
c.f.,
https://imagemagick.org/script/comma....php#intensity
The actual Forma_Gray.png linked earlier on GH was actually grayscaled via GIMP, which is not how I usually do it in my full pipeline, but is close enough
.
EDIT: Added the GIMP grayscale, and my usual full pipeline result from the full-color one to a "perfect" H2O result.