Append ingest processor stringifies arrays instead of processing the elements one by one

[Technici4n]

Hello, it seems that the append processor "stringifies" input arrays instead of processing the elements one by one. Could that be? (Using elasticsearch 8.6.2)

Pipeline:

    "description": "testing issues with append",
    "processors": [
      {
        "append": {
          "field": "copy_to",
          "value": [
            "{{{from}}}"
          ],
          "ignore_failure": true
        }
      }
    ]

Test document:

[
  {
    "_index": "o",
    "_id": "i",
    "_source": {
      "from": [
        2,
        3
      ]
    }
  }
]

Simulated output:

{
  "docs": [
    {
      "doc": {
        "_index": "o",
        "_id": "i",
        "_version": "-3",
        "_source": {
          "copy_to": [
            "{0=2, 1=3}"
          ],
          "from": [
            2,
            3
          ]
        },
        "_ingest": {
          "timestamp": "2023-05-15T16:11:17.691991822Z"
        }
      }
    }
  ]
}

I'd expect copy_to to be [2, 3], not {0=2, 1=3}.