Split valuation but category is grey in one material type

hi,

i met a strange things about field "valuation category" in material master. in our plant, we active split valuation, and all the setting is ok.

but when i create a material with material type FERT, the field valuation category is grey, can't enter value, but if i changed the material type, then can enter the category value. i check the field in field group are all the same setting that is option by T-CODE, industry, field reference, plant and etc.

so i don't konw why one type can enter value but another type is grey?