Setting parse_origination_date doesn't update index's lifecycle date

[magnusbaeck (Magnus Bäck)]

I have an old index that's been reindexed and I want my ILM policy to work as if the reindexing never happened. I expected setting the index.lifecycle.parse_origination_date setting to true would've taken care of that for me, but it appears it won't.

The index name is rndtools_logs_public_long-2019.08.16-000003 so I'd like the lifecycle date to be 2019-08-16 and hence the index age at the time of writing to be slightly less than 10 months.

PUT /rndtools_logs_public_long-2019.08.16-000003/_settings
{"index.lifecycle.parse_origination_date": true}

{"acknowledged":true}

The setting was accepted (though converted to a string):

GET /rndtools_logs_public_long-2019.08.16-000003/_settings

{
  "rndtools_logs_public_long-2019.08.16-000003": {
    "settings": {
      "index": {
        "lifecycle": {
          "name": "rndtools_logs_long",
          "parse_origination_date": "true",
          "indexing_complete": "true",
          "rollover_alias": "rndtools_logs_public_long"
        },
        "refresh_interval": "5s",
        "number_of_shards": "1",
        "blocks": {
          "write": "true"
        },
        "provided_name": "rndtools_logs_public_long-2019.08.16-000003",
        "creation_date": "1582202339997",
        "number_of_replicas": "1",
        "uuid": "ydMulx6_S3eMToRb7Bn4HQ",
        "version": {
          "created": "7040299"
        }
      }
    }
  }
}

Yet if I ask ILM about the lifecycle date and the index age they're not what I expect (inserted epoch translations as # comments for convenience):

GET /rndtools_logs_public_long-2019.08.16-000003/_ilm/explain                                                                                                                  

{
  "indices": {
    "rndtools_logs_public_long-2019.08.16-000003": {
      "index": "rndtools_logs_public_long-2019.08.16-000003",
      "managed": true,
      "policy": "rndtools_logs_long",
      "lifecycle_date_millis": 1592902376161,  # 2020-06-23 08:52Z
      "age": "1.83h",
      "phase": "warm",
      "phase_time_millis": 1592902376745,  # 2020-06-23 08:52Z
      "action": "complete",
      "action_time_millis": 1592903674807,  # 2020-06-23 09:14Z
      "step": "complete",
      "step_time_millis": 1592903674807,  # 2020-06-23 09:14Z
      "phase_execution": {
        "policy": "rndtools_logs_long",
        "phase_definition": {
          "min_age": "0ms",
          "actions": {
            "forcemerge": {
              "max_num_segments": 1
            }
          }
        },
        "version": 3,
        "modified_date_in_millis": 1586342084210  # 2020-04-08 10:34Z
      }
    }
  }
}

This is on ES 7.6.1. Does anyone know what's going on?