There's currently a bug when `returnStoredSource: true` is added as a field to a `$search` inside of a `$lookup`:

```
$search is only valid as the first stage in a pipeline
```

 Users should be allowed to add this flag to any of their `$search` queries. `returnStoredSource` works with `$unionWith` already so a good place to start investigating this issue would be to see how this feature works with that stage.