Trying to parital update a doc but getting error regarding date field with epoch_second format

I'm trying to partially update an existing document which is already in the index and is indexed well, meaning I can view it in Kibana which uses timestamp field to display the documents. I'm trying to update only the doc's name of id test_id my request is:

POST shirkan_test/_update/test_id
{
  "doc": {
    "name": "shirkan"
  },
  "doc_as_upsert": true
}

Getting the following error:

{
  "error": {
    "root_cause": [
      {
        "type": "mapper_parsing_exception",
        "reason": "failed to parse field [timestamp] of type [date] in document with id 'test_id'. Preview of field's value: '1.602505857664299E9'"
      }
    ],
    "type": "mapper_parsing_exception",
    "reason": "failed to parse field [timestamp] of type [date] in document with id 'test_id'. Preview of field's value: '1.602505857664299E9'",
    "caused_by": {
      "type": "illegal_argument_exception",
      "reason": "failed to parse date field [1.602505857664299E9] with format [epoch_second]",
      "caused_by": {
        "type": "date_time_parse_exception",
        "reason": "Failed to parse with all enclosed parsers"
      }
    }
  },
  "status": 400
}

Much appreciate any help with this. Thanks.

EDIT: adding index mapping

{
  "mapping": {
    "properties": {
      "condition": {
        "type": "text",
        "index": false
      },
      "name": {
        "type": "text",
        "fields": {
          "keyword": {
            "type": "keyword",
            "ignore_above": 256
          }
        }
      },
      "rank": {
        "type": "double"
      },
      "timestamp": {
        "type": "date",
        "format": "epoch_second"
      }
    }
  }
}

EDIT 2: Changing timestamp format to strict_date_optional_time_nanos doesn't yield such an error and the upsert works well.

Looks like for now, the solution which worked for me to this problem is to change the timestamp field format from epoch_second to strict_date_optional_time_nanos. Other formats may work as well. I had to completely delete the index and recreate it since I came across the same error message when trying to re-index. As mentioned in one of my comments, I filed a bug report here: https://github.com/elastic/elasticsearch/issues/64050