Script_score query: fvh highlighter doesn't work

Hi,

(ElasticSearch 7.8.0)

it looks like script_score queries in conjunction with fvh highligher and matched_fields doesn't work. I don't know if i'm missing something, but i couldn't find any do's/don'ts in the documentation reg. this:

Mapping:

PUT test
{
  "settings": {
    "index": {
      "max_ngram_diff": 20
    },
    "analysis": {
      "filter": {
        "ngram_2_to_12": {
          "type": "ngram",
          "min_gram": "2",
          "max_gram": "12"
        }
      },
      "analyzer": {
        "my_lowercase": {
          "type": "custom",
          "tokenizer": "standard",
          "filter": [
            "lowercase"
          ]
        },
        "my_ngram": {
          "type": "custom",
          "tokenizer": "standard",
          "filter": [
            "lowercase",
            "ngram_2_to_12"
          ]
        }
      }
    }
  },
  "mappings": {
    "properties": {
      "message": {
        "type": "text",
        "term_vector": "with_positions_offsets",
        "fields": {
          "ngram": {
            "type": "text",
            "term_vector": "with_positions_offsets",
            "analyzer": "my_ngram",
            "search_analyzer": "my_lowercase"
          }
        }
      },
      "timestamp": {
        "type": "date",
        "format": "date_time"
      }
    }
  }
}

Docs:

POST test/_doc
{
  "message": "just a test please ignore",
  "timestamp": "2020-09-17T06:49:20.488Z"
}
POST test/_doc
{
  "message": "testing again",
  "timestamp": "2020-09-16T12:00:00.0Z"
}

Works as expected using multi_match query:

GET /test/_search
{
    "query": {
      "multi_match": {
        "query": "es",
        "fields": ["message", "message.ngram"]
      }
    },
    "highlight": {
      "number_of_fragments": 1,
      "fields": {
        "message": {
          "type": "fvh",
          "matched_fields": ["message", "message.ngram"]
        }
      }
    }
}

Doesn't work when switching to script_score query - no highlights are returned:

GET /test/_search
{
    "query": {
      "script_score": {
        "query": {
          "multi_match": {
            "query": "es",
            "fields": ["message", "message.ngram"]
          }
        },
        "script": {
          "source": "1"
        }
      }
    },
    "highlight": {
      "number_of_fragments": 1,
      "fields": {
        "message": {
          "type": "fvh",
          "matched_fields": ["message", "message.ngram"]
        }
      }
    }
}

It doesn't matter if searching for "test" or "es", in either case no highlights are returned.

Update: upgrading to 7.9.1 didn't solve the issue