Flow type grandchild inheritance issues

Question

I'm curious as to why this inheritance model succeeds between a base and child, but fails between a child and grandchild. I'm sure there is something I've missed here. The error would make sense to me, if only it also occurred between base and child.

type BaseType = {
    base: true,
    ...
}

type fnType<O: BaseType> = (options: O) => void;

const base = (options: BaseType) => {
    return;
}

function extend<T: BaseType>(parent: fnType<T>): fnType<T>{
    return (options: T): void => {
        return parent(options);
    }
}

type ChildOneType = {
    ...BaseType,
    one: true,
    ...
};

const childOne = extend<ChildOneType>(base);

type ChildTwoType = {
    ...ChildOneType,
    two: true,
    ...
};

// why fail?
// ---
// Cannot call `extend` with `childOne` bound to `parent` because property `two`
// is missing in `ChildOneType` [1] but exists in `ChildTwoType` [2] in type
// argument `O` [3]. [prop-missing]
const childTwo = extend<ChildTwoType>(childOne);

TryFlow