[CI] ES92GpuHnswWriteGraphTests testCpuFallbackGraphMatchesLucene fails with seed EA6DA5BD8CC04BF6

[KevinCorcoran]

CI Link

https://gradle-enterprise.elastic.co/s/u2nvwxvnxshla/tests/task/:libs:gpu-codec:test/details/org.elasticsearch.gpu.codec.ES92GpuHnswWriteGraphTests/testCpuFallbackGraphMatchesLucene

Repro line

./gradlew ":libs:gpu-codec:test" --tests "org.elasticsearch.gpu.codec.ES92GpuHnswWriteGraphTests.testCpuFallbackGraphMatchesLucene" -Dtests.seed=EA6DA5BD8CC04BF6 -Dtests.locale=bg-BG -Dtests.timezone=Asia/Dushanbe

Does it reproduce?

Only on Linux x86_64 (AVX-2): 3/3 attempts in CI failed identically with this seed (Buildkite retries reuse the build's seed). The same seed passes on macOS/aarch64, consistent with the SIMD analysis in #149659.

Applicable branches

main

Failure history

Appears to be a recurrence of #149659 (closed 2026-05-26) with a different seed. No other failures of this test in the last 7 days per ci-stats — the failing build is an unrelated docs-only PR (#154901) that drew this seed:

• https://buildkite.com/elastic/elasticsearch-pull-request/builds/166785
• Build scans: https://gradle-enterprise.elastic.co/s/u2nvwxvnxshla, https://gradle-enterprise.elastic.co/s/uo3ibyl2epfri, https://gradle-enterprise.elastic.co/s/rqmpm32xwp7so

Failure excerpt

java.lang.AssertionError: kNN results for query 35 should match expected:<[0, 161, 35, 214, 198, 8, 249, 201, 58, 235]> but was:<[0, 161, 35, 214, 198, 8, 185, 249, 58, 235]>
  at org.elasticsearch.gpu.codec.ES92GpuHnswWriteGraphTests.assertSearchResultsEqual(ES92GpuHnswWriteGraphTests.java:178)

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[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)