Setting RIg to "humanoid" changes materials IDs

I exported some characters that have a few materials assigned to them (skin, hair, outfit, weapon…).
In Unity, I re-assigned a few materials to the character’s mesh.
Now in “import settings” I change the rig type from “none” to “humanoid” and all the materials ID are reverted to the default of the FBX.

Why is that? How can I set the rig to “humanoid” and keep all the materials IDs as is (because I spent days doing that)

bump!