My post was answered in the Mongo DB Community Forum.

https://www.mongodb.com/community/forums/t/atlas-search-across-enbedded-document-and-root-document-fields-using-compound-must/303015

Palyground with solved solution

https://search-playground.mongodb.com/tools/code-playground/snapshots/672a21d77816de283aa55341

This issue was solved by using the below search aggregation

Search

[
  {
    "$search": {
      "index": "default",
      "compound": {
        "must": [
          {
            "text": {
              "query": "big",
              "path": {
                "wildcard": "*"
              }
            }
          },
          {
            "text": {
              "query": "piano",
              "path": {
                "wildcard": "*"
              }
            }
          }
        ]
      }
    }
  }
]