Autocomplete search in Array of Objects MongoDB

The objective is to perform an Autocomplete type search on a field under Array of Objects:

"hashTags": [ 
    {
        "tag": "Smartwatch",
        "position": 1
    },
    {
        "tag": "Bluetooth",
        "position": 2
    } 
]

i.e my autocomplete search should work on hashTags.tag path. Already tried running Aggregate search query as below:

{
  $search: {
    index: 'hashtags',
    autocomplete: {
      path: 'hashTags.tag',
      query: term,
    },
  },
}

And having created Search index in Mongo at supposed Collection as below:

{
  "analyzer": "lucene.standard",
  "searchAnalyzer": "lucene.standard",
  "mappings": {
    "dynamic": false,
    "fields": {
      "hashTags": {
        "fields": {
          "tag": {
            "analyzer": "lucene.standard",
            "type": "autocomplete"
          }
        },
        "type": "document"
      }
    }
  }
}

But no search result so far, mongo returns 0 results. Is autocomplete search even viable on Array of objects in MongoDb? Please guide if there could be any possible syntax issues while creating Search Index in Mongo.