Help debugging: Expected MIME-Type to be 'application/javascript'... but got '(null)'

Do you want to request a feature or report a bug?

Report a bug and request help debugging.

What is the current behavior?

When running a React Native 0.64 app with Hermes on a physical iOS device, I eventually hit an error while trying to load the app JS, which says Expected MIME-Type to be 'application/javascript' or 'text/javascript', but got '(null)'.

Clearing out node_modules and running with --reset-cache does fix the issue (edit: just --reset-cache is sufficient, no need to delete node_modules), but it doesn't take long to occur again. It's inconsistent, but I've even seen it fail on the first refresh after a reset-cache launch.

I haven't seen this on an Android device, or on an iOS simulator. Android continues to be able to load JS from metro after this happens.

If the current behavior is a bug, please provide the steps to reproduce and a minimal repository on GitHub that we can yarn install and yarn test.

I unfortunately have no idea how to reproduce this. If anyone could provide debugging suggestions, I'm happy to run them.

What is the expected behavior?

The javascript is served so that the app can load it 😄

Please provide your exact Metro configuration and mention your Metro, node, yarn/npm version and operating system.

Metro version: 0.64.0
Node version: v14.16.0
yarn version: 1.22.10

metro.config.js:

const {getDefaultConfig} = require('metro-config');

module.exports = (async () => {
  const {
    resolver: {sourceExts, assetExts},
  } = await getDefaultConfig();
  return {
    transformer: {
      getTransformOptions: async () => ({
        transform: {
          experimentalImportSupport: false,
          inlineRequires: true,
        },
      }),
      babelTransformerPath: require.resolve('react-native-svg-transformer'),
    },
    resolver: {
      assetExts: assetExts.filter(ext => ext !== 'svg'),
      sourceExts: [...sourceExts, 'svg'],
    },
  };
})();

Start with the supplied metro.config.js, Metro 0.64.0 environment details, and the physical iOS device setup using Hermes. Reproduce the intermittent '(null)' MIME-type response around refreshes and compare behavior after --reset-cache with Android and the iOS simulator. Done means identifying the cause and confirming that Metro consistently serves the JavaScript so the app loads.