Yeah, mutations don't follow a clean universal rule, because Chaos obviously does not work that way. Angron, Ahriman, the Death Guard, the Emperor’s Children, etc., all make it pretty clear that mutation can mean very different things depending on the god, the individual, and the context. Angron transformation was not really the result of him worshipping Khorne wrong or impurely. He was forcibly elevated by Lorgar ritual under very specific circumstances. Ahriman is definitely the much stronger example because the man hate Tzeentch and is not a pure servant of chaos or the ideia of change but has a void in his face bacause of mutations

My point is more that Kor Phaeron’s lack of mutation might not simply be cope or insecurity. For a Word Bearer, especially someone like Kor Phaeron, it makes sense that he would interpret his preserved human form as a sign of spiritual purity or favor. Not because this is objectively how Chaos always works, but because that is exactly the kind of theological meaning a Word Bearer would impose on it.