Optional tags are lost after merge

[asins]

interface A {
  t1?: number;
  t2?: number;
  t3?: number;
  t4?: number;
}

type B = Pick<A, 't2' | 't4'> & {
  hostList: string[];
};

When the Merge method is executed

type MergeB = Merge<Pick<A, 't2' | 't4'>, {
  hostList: string[];
}>;

the desired result is:

type MergeB = {
    t2?: number;
    t4?: number;
    hostList: IRegionPoint[];
}

The actual result is:

type MergeB = {
    t2?: number;
    t4?: number;
    hostList?: IRegionPoint[];  // <--- Not what I expected
}

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