HMR generally works fine. However, when I modify my index.js file, it doesn't reload the components defined within the file. Here's how to reproduce this behaviour:

• Create a new react app
• replace the index.js file

import React from "react"
import ReactDOM from "react-dom"

function Foo() {
  return "first version"
}

ReactDOM.render(
  <React.StrictMode>
    <Foo />
  </React.StrictMode>,
  document.getElementById("root")
)

Load the page and you should see first version

Change the first version string to anything else. The page doesn't change since the component doesn't get reloaded.