I do not believe there is some abstract form of "humanity" outside of people desperately rationalizing their existence in the Hellhole Malus is. There is only the human norm and various deviations from it.
In short I do not think there is a qualitative difference between the mutations of the spirit suffered by a Magister of the Orders and a warrior priest of Sigmar.
The origin of Divine Lore lies with whatever race the god belongs to. Say in case of a human god.
Human -> Faith -> God -> Divine Lore -> Mutation of the Spirit
Mutations of the spirit take shape related to the relevant god, because the relevant god is a construct by humans, the mutations are closer to the default human that isn't touched by either Lores than someone who would have his spirit mutated by a Wind of Magic.
Say, for a hypothetical case, a human worships a halfling god and get's his spirit mutated.
Halfling -> Faith -> God -> Divine Lore -> Mutation of the Spirit
Assuming that said god is primarily worshipped by halflings and perceived as a halfling. A mutation on the spirit would move the human away from "human" and more to the perceived nature of said god. Which would be halfling in this case. While he/she wouldn't turn into a halfling or something like that, he/she would take on either physical or mental traits normally associated with the Halfling god and therefore halflings.
With the Winds of Magic it's different. It's origins lie in the Aether itself.
________________________-> Hysh -> Lore of Light -> Mutation of the Spirit
Aether -> Vortex -> Winds -> Shyish - Lore of Death -> Mutation of the Spirit
________________________-> Azyr - Lore of the Heavens -> Mutation of the Spirit
________________________ect
In this case the origin of the mutation would be the individual wind. As with the previous examples the mutation is influenced by it's origins, however since the winds are more comparable to forces of nature than to a relatable concept. The mutation isn't "blunted" like with Divine magic since the origin lies so far away from human understanding.
Instead of adding "Idealized Human" to "default Human" and mutating the soul relatively little. The Wind of Magic add a concept that is too different from "Human" to the soul. Therefor people who have their soul mutated by said wind, are changed relatively more and lose more on the "Humanity" scale.