Issue with DeepMutable and branded types in TypeScript

type a = Types.DeepMutable<"abc" & { brand: "abc" }>

type a = {
    [x: number]: string;
    toString: () => string;
    charAt: (pos: number) => string;
    charCodeAt: (index: number) => number;
    concat: (...strings: string[]) => string;
    indexOf: (searchString: string, position?: number) => number;
    ... 45 more ...;
    brand: "abc";
}