This looks to be a newly added test in #89430 and failed for me in a PR.

**Build scan:**
https://gradle-enterprise.elastic.co/s/ctyyqap6ywxok/tests/:server:test/org.elasticsearch.index.shard.IndexShardTests/testFlushOnIdleConcurrentFlushDoesNotWait

**Reproduction line:**
`./gradlew ':server:test' --tests "org.elasticsearch.index.shard.IndexShardTests.testFlushOnIdleConcurrentFlushDoesNotWait" -Dtests.seed=3B825C4D0772201A -Dtests.locale=ru-RU -Dtests.timezone=Australia/Broken_Hill -Druntime.java=17`

**Applicable branches:**
main

**Reproduces locally?:**
No

**Failure history:**
https://gradle-enterprise.elastic.co/scans/tests?tests.container=org.elasticsearch.index.shard.IndexShardTests&tests.test=testFlushOnIdleConcurrentFlushDoesNotWait

**Failure excerpt:**
```
java.lang.AssertionError: expected to see should see first flush getting lock immediately but did not
Expected: <true>
     but: was <false>

  at __randomizedtesting.SeedInfo.seed([3B825C4D0772201A:B93D2E3091554306]:0)
  at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:18)
  at org.elasticsearch.test.MockLogAppender$SeenEventExpectation.assertMatched(MockLogAppender.java:121)
  at org.elasticsearch.test.MockLogAppender.assertAllExpectationsMatched(MockLogAppender.java:55)
  at org.elasticsearch.test.ESTestCase.assertBusy(ESTestCase.java:1104)
  at org.elasticsearch.test.ESTestCase.assertBusy(ESTestCase.java:1077)
  at org.elasticsearch.index.shard.IndexShardTests.testFlushOnIdleConcurrentFlushDoesNotWait(IndexShardTests.java:3939)
  at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(NativeMethodAccessorImpl.java:-2)
  at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:77)
  at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:568)
  at com.carrotsearch.randomizedtesting.RandomizedRunner.invoke(RandomizedRunner.java:1758)
  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.apache.lucene.tests.util.TestRuleSetupTeardownChained$1.evaluate(TestRuleSetupTeardownChained.java:44)
  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$1.evaluate(TestRuleMarkFailure.java:44)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl$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.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$1.evaluate(TestRuleIgnoreTestSuites.java:47)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:390)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl.lambda$forkTimeoutingTask$0(ThreadLeakControl.java:850)
  at java.lang.Thread.run(Thread.java:833)

```
server/src/test/java/org/elasticsearch/index/shard/IndexShardTests.java
});
        cancellingThread.start();
        cyclicBarrier.await();
        relocationThread.join();
        cancellingThread.join();
        if (shard.isRelocatedPrimary()) {
            logger.debug("shard was relocated successfully");
            assertThat(cancellingException.get(), instanceOf(IllegalIndexShardStateException.class));
            assertThat("current routing:" + shard.routingEntry(), shard.routingEntry().relocating(), equalTo(true));
            assertThat(relocationException.get(), nullValue());
        } else {
            logger.debug("shard relocation was cancelled");
            assertThat(
                relocationException.get(),
                either(instanceOf(IllegalIndexShardStateException.class)).or(instanceOf(IllegalStateException.class))
            );
            assertThat("current routing:" + shard.routingEntry(), shard.routingEntry().relocating(), equalTo(false));
            assertThat(cancellingException.get(), nullValue());
        }
        closeShards(shard);
    }
    public void testRelocateMissingTarget() throws Exception {
        final IndexShard shard = newStartedShard(true);
        final ShardRouting original = shard.routingEntry();
        final ShardRouting toNode1 = ShardRoutingHelper.relocate(original, "node_1");
        IndexShardTestCase.updateRoutingEntry(shard, toNode1);
        IndexShardTestCase.updateRoutingEntry(shard, original);
        final ShardRouting toNode2 = ShardRoutingHelper.relocate(original, "node_2");
        IndexShardTestCase.updateRoutingEntry(shard, toNode2);
        final AtomicBoolean relocated = new AtomicBoolean();