why polling of Apollo useQuery don't call onCompleted?

I am playing around the code: https://codesandbox.io/s/restless-framework-uldf4q?file=/src/App.js

import React, { Fragment } from "react";
import { gql } from "apollo-boost";
import { useQuery } from "@apollo/react-hooks";

const GET_DOG_PHOTO = gql`
  query Dog($breed: String!) {
    dog(breed: $breed) {
      id
      displayImage
    }
  }
`;

const breed = "dingo";

const App = () => {
  const [count, setCount] = React.useState(0);
  const { loading, error, data, startPolling, stopPolling } = useQuery(
    GET_DOG_PHOTO,
    {
      variables: { breed },
      onCompleted: () => {
        setCount((count) => count + 1);
      }
    }
  );

  if (loading) {
    return <h2>loading</h2>;
  }
  if (error) {
    return <h2>Whoops</h2>;
  }

  return (
    <div>
      <h1> {count}</h1>
      <Fragment>
        <img
          alt="Cute Doggo"
          src={data.dog.displayImage}
          style={{ height: 500, width: 500 }}
        />
        <button onClick={() =>startPolling(500)}>Start</button>
        <button onClick={stopPolling}>Stop</button>
      </Fragment>
    </div>
  );
};

export default App;

In my code, I setCount to count+1 using React.useState in the onCompleted callback? why it just stop counting when polling?

so what is the mechanism here?

I can also observe that the onCompleted callback is not invoked as expected in the reproducer you provide.

In your CodeSandbox, you are using a 3-year-old deprecated version of the useQuery hook. You could migrate to the latest @apollo/client package, which will solve the issue.

See this migrated CodeSandbox: https://codesandbox.io/s/apolllo-on-completed-n9wzge