Hybrid query highlight lexical matches

OpenSearch
, ,
1

Versions (relevant - OpenSearch/Dashboard/Server OS/Browser): Opensearch version 3.2.0

Describe the issue: Lexical highlight not returned for hybrid query
I have a knn index and I want to use a hybrid query while still keeping track of the lexical matches but Opensearch seems to ignore the highlight parameter.

Here is an example of the query structure:

GET my_index/_search?search_pipeline=hybrid_search_pipeline
{
  "_source": {
    "excludes": [
      "chunk_embedding"
    ]
  },
  "collapse": {
    "field": "id"
  },
  "highlight": {
    "fragment_size": 100,
    "post_tags": [
      "</span>"
    ],
    "pre_tags": [
      """<span solrfound="true" style="background:#fdfdb0;">"""
    ],
    "fields": {
      "description": {},
      "altDescription": {}
    }
  },
  "query": {
    "hybrid": {
      "queries": [
        {
          "query_string": {
            "fields": [
              "description^2.0",
              "altDescription^1.5",
              "attachmentData"
            ],
            "query": "rossini~1"
          }
        },
        {
          "neural": {
            "chunk_embedding": {
              "query_text": "rossini"
            }
          }
        }
      ]
    }
  },
  "size": 50
}

The search does not fail and the results are returned but there is no “highlight” element.
Searching similar topics I found some discussing semantic highlighting but that is NOT my need for now. I only need to obtain the lexical matches with highlight.
I also tried adding the “highlight_query” parameter but it did not solve the issue.

Configuration: Knn index. Both fields I am trying to use highlighting on have the mapping:

{
      "store": true,
      "term_vector": "yes",
      "type": "text"
    }

Can anyone help me?