Yup schema - `when` method is not working

Recently I am suffering with creating yup schema. Since I am doing it I see that the method when() totally doesn't work for me like it should as documentation said and other solutions I found on the internet. When my checkbox is checked to true all the fields on the schema should be required, but they don't. As seen in my example I tried three ways, and none of them is working. Maybe there is someone who knows what am I doing wrong?

My test code is:

import "./styles.css";
import * as yup from "yup";
import { FormProvider, useForm } from "react-hook-form";
import { yupResolver } from "@hookform/resolvers/yup";
import React, { useEffect } from "react";

export default function App() {
  const schema = yup.object({
    isRequired: yup.bool(),
    firstName: yup.string().when("isRequired", (_, schema) => {
      return schema.required();
    }),
    lastName: yup.string().when("isRequired", () => {
      return yup.string().required();
    }),
    contact: yup.string().when("isRequired", {
      is: true,
      then: yup.string().required("Required")
    })
  });

  const methods = useForm({
    mode: "all",
    defaultValues: {
      isRequired: true,
      firstName: "",
      lastName: "",
      contact: ""
    },
    resolver: yupResolver(schema),
    shouldUnregister: true
  });

  const { register, watch } = methods;
  const isRequired = watch("isRequired");

  useEffect(() => {
    console.log(schema.fields);
  }, [isRequired, schema]);

  return (
    <FormProvider {...methods}>
      <form autoComplete="off" noValidate>
        <input type="checkbox" {...register("isRequired")} />
        <input {...register("firstName")} />
        <input {...register("lastName")} />
        <input {...register("contact")} />
      </form>
    </FormProvider>
  );
}

https://codesandbox.io/s/infallible-wildflower-dsyon