Repository: create-react-app
Issue: Build fails with error 'jsx' must be in scope when using JSX but DISABLE_NEW_JSX_TRANSFORM flag is enabled
Labels: needs triage,issue: bug report

### Describe the bug

Up until today I was using react 16.9.0, CRA 3.x.x and emotionjs making use of their jsx pragma.
Today I upgraded in this order: CRA to 4.x.x first and React to 17.x.x second.

While I was on CRA 4 and react 16 I had no issues. After upgrading to React 17 sometimes, it's quite random, the build process fails, with the error `'jsx' must be in scope when using JSX ` (see bellow).

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Yes

### Which terms did you search for in User Guide?

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"Disable JSX transform"

### Environment

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```bash
Environment Info:

  current version of create-react-app: 4.0.1
  running from /home//.npm/_npx/862177/lib/node_modules/create-react-app

  System:
    OS: Linux 5.4 Ubuntu 20.04.1 LTS (Focal Fossa)
    CPU: (4) x64 Intel(R) Core(TM) i5-6600K CPU @ 3.50GHz
  Binaries:
    Node: 12.18.3 - ~/.nvm/versions/node/v12.18.3/bin/node
    Yarn: 1.22.5 - /usr/bin/yarn
    npm: 6.14.6 - ~/.nvm/versions/node/v12.18.3/bin/npm
  Browsers:
    Chrome: Not Found (I use chromium)
    Firefox: 84.0
  npmPackages:
    react: ^17.0.1 => 17.0.1 
    react-dom: ^17.0.1 => 17.0.1 
    react-scripts: 4.0.1 => 4.0.1 
  npmGlobalPackages:
    create-react-app: Not Found
```
Also, Typescript version 4.1.3

### Steps to reproduce

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1. Enable the new [`DISABLE_NEW_JSX_TRANSFORM`](https://create-react-app.dev/docs/advanced-configuration#__docusaurus:~:text=DISABLE_NEW_JSX_TRANSFORM) flag in the project's `.env` file, by `DISABLE_NEW_JSX_TRANSFORM=true`
2. Make use of emotion's `jsx` by [setting the pragma](https://emotion.sh/docs/css-prop#jsx-pragma) in order for the css prop to work without needing to eject
3. Compile the project

### Actual behavior

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Most of the times the compiling fails, with the following error

```bash
'jsx' must be in scope when using JSX                                                                 react/react-in-jsx-scope
The css prop can only be used if jsx from @emotion/react is imported and it is set as the jsx pragma  @emotion/jsx-import
```
![image](https://user-images.githubusercontent.com/3789226/102826526-80117200-43e9-11eb-87e8-7c615b0eae7c.png)


The **strange** thing is that this happens *sometimes*. Triggering another build *may* actually result in a successful compilation.

### Expected behavior

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I am aware of the fact that currently there are some issues between emotion and the new JSX transform, but my understanding is that enabling the `DISABLE_NEW_JSX_TRANSFORM` flag should actually make the project build/behave like in react versions <16.14.0 ?

Related issues:
https://github.com/system-ui/theme-ui/issues/1160#issuecomment-715530924
https://github.com/emotion-js/emotion/issues/2041

### Demo

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This demo reflects the important parts of the configuration in my project, however I could not reproduce it there.
https://codesandbox.io/s/awesome-browser-mzv6u?file=/src/App.js

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