User blog comment:B214/Shin DxD 1 discussion/@comment-26709288-20180814233035/@comment-1605975-20180816213406

I think it was mentioned before that only one out ten Devils get a Mutation Piece in their Evil Piece set, and that its appearance is not related to potential or strength, but rather pure chance. Rias just happened to have a Mutated Bishop and Sairaorg happened to have a Mutated Pawn. So perhaps Issei won't have one.