knn search over nested dense_vector fields returns fewer than k hits

[ChrisHegarty]

Several kNN search tests over nested fields are broken.

Reproduction is most straightforward when there is more than one segment, e.g. apply this small change to ensure that the index has more than one segment, then run the test.

$ git diff rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/100_knn_nested_search.yml
diff --git a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/100_knn_nested_search.yml b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/100_knn_nested_search.yml
index d627be2fb15..832767a53c4 100644
--- a/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/100_knn_nested_search.yml
+++ b/rest-api-spec/src/yamlRestTest/resources/rest-api-spec/test/search.vectors/100_knn_nested_search.yml
@@ -54,6 +54,12 @@ setup:
           - paragraph_id: 3
             vector: [0, 1.0, 0, 1.8, -15.0]
 
+  - do:
+      indices.refresh: {}
+  - do:
+      indices.flush:
+        index: test
+
   - do:
       index:
         index: test

$ ./gradlew ":rest-api-spec:yamlRestTest" --tests "org.elasticsearch.test.rest.ClientYamlTestSuiteIT" -Dtests.method="test {yaml=search.vectors/100_knn_nested_search/nested kNN search only}" -Druntime.java=24 --info 

REPRODUCE WITH: ./gradlew ":rest-api-spec:yamlRestTest" --tests "org.elasticsearch.test.rest.ClientYamlTestSuiteIT" -Dtests.method="test {yaml=search.vectors/100_knn_nested_search/nested kNN search only}" -Dtests.seed=BA82B9A7A9611F78 -Dtests.locale=sw -Dtests.timezone=America/Argentina/Ushuaia -Druntime.java=24

ClientYamlTestSuiteIT > test {yaml=search.vectors/100_knn_nested_search/nested kNN search only} FAILED
    java.lang.AssertionError: Failure at [search.vectors/100_knn_nested_search:94]: 
    Expected: <2>
         but: was <1>
        at __randomizedtesting.SeedInfo.seed([BA82B9A7A9611F78:32D6867D079D7280]:0)
        at org.elasticsearch.test.rest.yaml.ESClientYamlSuiteTestCase.executeSection(ESClientYamlSuiteTestCase.java:524)
        at org.elasticsearch.test.rest.yaml.ESClientYamlSuiteTestCase.test(ESClientYamlSuiteTestCase.java:484)
        at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
        at java.base/java.lang.reflect.Method.invoke(Method.java:565)
        at com.carrotsearch.randomizedtesting.RandomizedRunner.invoke(RandomizedRunner.java:1763)
        at com.carrotsearch.randomizedtesting.RandomizedRunner$8.evaluate(RandomizedRunner.java:946)
        at com.carrotsearch.randomizedtesting.RandomizedRunner$9.evaluate(RandomizedRunner.java:982)
        at com.carrotsearch.randomizedtesting.RandomizedRunner$10.evaluate(RandomizedRunner.java:996)
        at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
        at org.junit.rules.RunRules.evaluate(RunRules.java:20)
        at org.apache.lucene.tests.util.TestRuleSetupTeardownChained$1.evaluate(TestRuleSetupTeardownChained.java:48)
        at org.apache.lucene.tests.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
        at org.apache.lucene.tests.util.TestRuleThreadAndTestName$1.evaluate(TestRuleThreadAndTestName.java:45)
        at org.apache.lucene.tests.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:60)
        at org.apache.lucene.tests.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:44)
        at org.junit.rules.RunRules.evaluate(RunRules.java:20)
        at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
        at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:390)
        at com.carrotsearch.randomizedtesting.ThreadLeakControl.forkTimeoutingTask(ThreadLeakControl.java:843)
        at com.carrotsearch.randomizedtesting.ThreadLeakControl$3.evaluate(ThreadLeakControl.java:490)
        at com.carrotsearch.randomizedtesting.RandomizedRunner.runSingleTest(RandomizedRunner.java:955)
        at com.carrotsearch.randomizedtesting.RandomizedRunner$5.evaluate(RandomizedRunner.java:840)
        at com.carrotsearch.randomizedtesting.RandomizedRunner$6.evaluate(RandomizedRunner.java:891)
        at com.carrotsearch.randomizedtesting.RandomizedRunner$7.evaluate(RandomizedRunner.java:902)
        at org.elasticsearch.test.cluster.local.DefaultLocalElasticsearchCluster$1.evaluate(DefaultLocalElasticsearchCluster.java:49)
        at org.apache.lucene.tests.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
        at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
        at org.apache.lucene.tests.util.TestRuleStoreClassName$1.evaluate(TestRuleStoreClassName.java:38)
        at com.carrotsearch.randomizedtesting.rules.NoShadowingOrOverridesOnMethodsRule$1.evaluate(NoShadowingOrOverridesOnMethodsRule.java:40)
        at com.carrotsearch.randomizedtesting.rules.NoShadowingOrOverridesOnMethodsRule$1.evaluate(NoShadowingOrOverridesOnMethodsRule.java:40)
        at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
        at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
        at org.apache.lucene.tests.util.TestRuleAssertionsRequired$1.evaluate(TestRuleAssertionsRequired.java:53)
        at org.apache.lucene.tests.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:43)
        at org.apache.lucene.tests.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:44)
        at org.apache.lucene.tests.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:60)
        at org.apache.lucene.tests.util.TestRuleIgnoreTestSuites$1.evaluate(TestRuleIgnoreTestSuites.java:47)
        at org.junit.rules.RunRules.evaluate(RunRules.java:20)
        at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
        at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:390)
        at com.carrotsearch.randomizedtesting.ThreadLeakControl.lambda$forkTimeoutingTask$0(ThreadLeakControl.java:850)
        at java.base/java.lang.Thread.run(Thread.java:1447)

        Caused by:
        java.lang.AssertionError: 
        Expected: <2>
             but: was <1>
            at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:20)
            at org.junit.Assert.assertThat(Assert.java:964)
            at org.junit.Assert.assertThat(Assert.java:930)
            at org.elasticsearch.test.rest.yaml.section.MatchAssertion.doAssert(MatchAssertion.java:100)
            at org.elasticsearch.test.rest.yaml.section.Assertion.execute(Assertion.java:68)
            at org.elasticsearch.test.rest.yaml.ESClientYamlSuiteTestCase.executeSection(ESClientYamlSuiteTestCase.java:504)
            ... 41 more

[elasticsearchmachine]

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