[CI] CcsCommonYamlTestSuiteIT test {p0=search.vectors/42_knn_search_int4_flat/Vector similarity with filter only} failing

[elasticsearchmachine]

Build Scans:

• elasticsearch-periodic #5491 / openjdk17_checkpart1_java-matrix
• elasticsearch-periodic #5391 / openjdk21_checkpart1_java-matrix

Reproduction Line:

./gradlew ":qa:ccs-common-rest:yamlRestTest" --tests "org.elasticsearch.test.rest.yaml.CcsCommonYamlTestSuiteIT" -Dtests.method="test {p0=search.vectors/42_knn_search_int4_flat/Vector similarity with filter only}" -Dtests.seed=D9203CD3FBF0E7A -Dtests.locale=to -Dtests.timezone=Asia/Bishkek -Druntime.java=17

Applicable branches:
8.17

Reproduces locally?:
N/A

Failure History:
See dashboard

Failure Message:

java.lang.AssertionError: Failure at [search.vectors/42_knn_search_int4_flat:220]: field [hits.hits] doesn't have length [1]
Expected: <1>
     but: was <0>

Issue Reasons:

• [8.17] 2 failures in test test {p0=search.vectors/42_knn_search_int4_flat/Vector similarity with filter only} (0.3% fail rate in 663 executions)
• [8.17] 2 failures in pipeline elasticsearch-periodic (9.5% fail rate in 21 executions)

Note:
This issue was created using new test triage automation. Please report issues or feedback to es-delivery.

[elasticsearchmachine]

Pinging @elastic/es-core-infra (Team:Core/Infra)

[elasticsearchmachine]

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

[benwtrent]

Like other ccs vector search failures, this is a data setup issue. Generally, this occurs when there is only one vector quantized in a given segment, which we try to protect against, but in CCS, it could be that a shard only gets a single vector.