I am trying to use an Ingestion pipeline to add _timestamp field to the documents.
While using the ingestion simulate API , the response contains the _timestamp field that gets added due to the pipeline processor
POST _ingest/pipeline/_simulate?verbose
{
  "pipeline": {
    "description": "Adds a timestamp field at the current time",
    "processors": [
      {
        "set": {
          "field": "_timestamp",
          "value": "{{_ingest.timestamp}}"
        }
      }
    ]
  },
  "docs":[
    {
      "_index":"newindex",
      "_type": "document",
      "_id":"1",
      "_source":{
        "example":"data3"
      }
    }
    ]
}
But after issuing a write request using the pipeline, the _timestamp field fails to get added to the document source. Changing the field name from _timestamp to timestamp seems to work though.