Typescript does not recognize the correct generic type with index access [duplicate]

Question

I am trying to process nested arrays of two different objects with the same type.

Playground

The type ArrayProperties returns the correct type definition.

type ArrayProperties<T> = {
    [P in keyof T as T[P] extends Array<any> ? P : never]: T[P];
}

I do not understand why Typescript won't recognize the correct types in the function body of concatArrays.

function concatArrays<Type extends object, Prop extends keyof ArrayProperties<Type>>
  (arr1: Type, arr2: Type, prop: Prop) {
    const a = arr1[prop];
    const b = arr2[prop];
    // Why are a and b of type any and not of type Array?
    return a.concat(b);  // `Property 'concat' does not exist on type 'Type[Prop]'.`
}

Example:

const arr1 = {
    bla: 'str',
    foo: [1, 2]
};

const arr2 = {
    bla: 'ing',
    foo: [3, 4]
};

const result = concatArrays(arr1, arr2, 'foo');
// result type should be number[]

const typeError = concatArrays(arr1, arr2, 'bla'); // type error => correct