`_refresh` API returns HTTP 400 with illegal_argument_exception about unknown segment generation

[carsonip]

Elasticsearch Version

8.15.4, but also seen in 9.0.1

Installed Plugins

No response

Java Version

bundled

OS Version

N/A, ECH

Problem Description

Any _refresh requests to a particular ES cluster on ECH returns HTTP 400

Request:

POST /partial-.ds-traces-apm.sampled-default-2025.05.22-001732,partial-.ds-traces-apm.sampled-default-2025.05.05-001693,partial-.ds-traces-apm.sampled-default-2025.05.10-001705,partial-.ds-traces-apm.sampled-default-2025.05.23-001735,partial-.ds-traces-apm.sampled-default-2025.05.26-001742,.ds-traces-apm.sampled-default-2025.06.05-001767,partial-.ds-traces-apm.sampled-default-2025.05.09-001702,.ds-traces-apm.sampled-default-2025.05.31-001754,partial-.ds-traces-apm.sampled-default-2025.05.28-001748,partial-.ds-traces-apm.sampled-default-2025.05.15-001717,partial-.ds-traces-apm.sampled-default-2025.05.18-001723,partial-.ds-traces-apm.sampled-default-2025.05.19-001726,partial-.ds-traces-apm.sampled-default-2025.05.07-001696,partial-.ds-traces-apm.sampled-default-2025.05.30-001751,.ds-traces-apm.sampled-default-2025.06.01-001757,partial-.ds-traces-apm.sampled-default-2025.05.17-001720,partial-.ds-traces-apm.sampled-default-2025.05.27-001745,partial-.ds-traces-apm.sampled-default-2025.05.14-001714,partial-.ds-traces-apm.sampled-default-2025.05.24-001738,.ds-traces-apm.sampled-default-2025.06.04-001764,partial-.ds-traces-apm.sampled-default-2025.05.20-001729,partial-.ds-traces-apm.sampled-default-2025.05.08-001699,partial-.ds-traces-apm.sampled-default-2025.05.13-001711,partial-.ds-traces-apm.sampled-default-2025.05.25-001739,.ds-traces-apm.sampled-default-2025.06.02-001761,partial-.ds-traces-apm.sampled-default-2025.05.12-001708/_refresh?ignore_unavailable=true

Response:

 "_shards": {
    "total": 52,
    "successful": 31,
    "failed": 21,
    "failures": [
      {
        "shard": 0,
        "index": "partial-.ds-traces-apm.sampled-default-2025.05.30-001751",
        "status": "BAD_REQUEST",
        "reason": {
          "type": "illegal_argument_exception",
          "reason": "Validation Failed: 1: segment generation is unknown;"
        }
      },
      {
        "shard": 0,
        "index": "partial-.ds-traces-apm.sampled-default-2025.05.10-001705",
        "status": "BAD_REQUEST",
        "reason": {
          "type": "illegal_argument_exception",
          "reason": "Validation Failed: 1: segment generation is unknown;"
        }
      },
...

Related issue for APM Server to better handle errors from _refresh: elastic/apm-server#17117

Steps to Reproduce

Unknown

Logs (if relevant)

No response

[elasticsearchmachine]

Pinging @elastic/es-distributed-indexing (Team:Distributed Indexing)