Not a webpack bundle unhandled rejection on every chunk

Multi entry webpack 4 (latest) configuration on Node 14.
It does this for every chunk

```
errored out during transformation Error: /Users/claudio/Projects/loom/js/404-3594367e1117c0d15bb5.js: Not a webpack bundle
at mod (/Users/claudio/Projects/loom/node_modules/optimize-plugin/dist/worker.js:203:15)
at /Users/claudio/Projects/loom/node_modules/optimize-plugin/dist/worker.js:211:9
at Array.forEach ()
at getWebpackModules (/Users/claudio/Projects/loom/node_modules/optimize-plugin/dist/worker.js:207:10)
at getWebpackBootstrap (/Users/claudio/Projects/loom/node_modules/optimize-plugin/dist/worker.js:85:25)
at /Users/claudio/Projects/loom/node_modules/optimize-plugin/dist/worker.js:256:29
at Array.forEach ()
at PluginPass.Program (/Users/claudio/Projects/loom/node_modules/optimize-plugin/dist/worker.js:255:26)
at newFn (/Users/claudio/Projects/loom/node_modules/optimize-plugin/node_modules/@babel/traverse/lib/visitors.js:175:21)
at NodePath._call (/Users/claudio/Projects/loom/node_modules/optimize-plugin/node_modules/@babel/traverse/lib/path/context.js:55:20) {
type: 'Error',
code: 'BABEL_TRANSFORM_ERROR'
}
```

Any pointers? 🙏