Hewed flesh does not proc at all against World Bosses.
It seems to be only semi-functional against other bosses.
Given the percentage chance it’s hard to test the semi-functional part, but it is absolutely not working properly all the time.
And as stated, against world bosses the proc-rate is literally zero. Not one corpse generated from thousands of hits.
It even says the chance is doubled against bosses, but this is clearly broken.