Parent method not accessible in the children instance after translation to JavaScript from TypeScript

Question

I've got the following TypeScript classes:

class AnyBackendRoute {
   public getProps() {...}
}

class AnyBFFRoute extends AnyBackendRoute {
   public getProps() {...}
}

class UserSignUpRoute extends AnyBFFRoute {
   public getSomethingElse() {...}
}

I transpile them to commonJS-ES6 and include them in another file (.js).

In the original code (in test file, before transpilation) I can easily do:

const route = new UserSignUpRoute()

// getProps() visible from AnyBFFRoute as it's a public method
console.log(route.getProps())

but when I try the same thing with the transpiled code (that contains also .d.ts file), I am getting a TypeScript error:

Type 'UserSignUpRoute' is missing the following properties from type 'AnyBFFRoute': getProps

In the transpiled code I would expect to find something like:

UserSignUpRoute.prototype.getProps = function () {
    return this.props;
};

but it's not there! Same, in the appropriate .d.ts file. I cannot see getProps() method from the parent class but only the local public methods.

Am I wrong to think those methods should be available in the UserSignUpRoute.js as well?

If so, why there is an inconsistency so that I can call route.getProps() in the pre-transpiled code and not afterwards?