Unexpected behavior of `keyof` with primitive types in TypeScript

type Moo<T> = { [K in keyof T]: "moo" };
type MooR = Moo<{j: string}>; // Ok: evals to {j: Moo}
type MooS = Moo<string>; // why MooS = string?
type MooN = Moo<number>; // why MooN = number?

type Moo<T> = { [K in keyof T]: "moo" };
type MooR = Moo<{j: string}>; // Ok: evals to {j: Moo}
type MooS = Moo<string>; // why MooS = string?
type MooN = Moo<number>; // why MooN = number?