Elasticsearch stores the full document indexed as _SOURCE. One can use it to project fields even though they are not marked as Store.YES in Hibernate Search.
But Elasticsearch also let people store the value of the field in Lucene itself (like Hibernate Search does). The reason is that it might be much faster to retrieve a small piece of stored info from Lucene vs loading the JSON document and filtering it out.

So we should map Hibernate Search Store.YES to the Elasticsearch's Lucene Store.YES.

That might be already done.