image optimizer for web

Dear Cedric,

there is an old problem with converting colors to shades of gray,
learn about it here: https://entropymine.com/imageworsener/grayscale/

For my part, I attach an archive with three images to illustrate the essence:
a) color image,
b) grayscale image produced by pingo 1.16,
c) grayscale image that is actually expected.

You will immediately notice the difference when looking at a red heart.

https://upl.io/download/lz37vk/pingo.grayscale.202401a.zip
Size: 128178 bytes
BLAKE3: 1ecaadbb6fb1eb62a797fae84de75629cc444d19dc0e154d783d399919469341

What is the difference between -grayscale and undocumented -grayline?
Different formulas?