Started failing in this way about 10 days ago (3-7-22). Seems to be affecting 7.17 and 8.1 branches.

**Build scan:**
https://gradle-enterprise.elastic.co/s/kg23spzlawfu6/tests/:x-pack:plugin:ilm:internalClusterTest/org.elasticsearch.xpack.ilm.ILMMultiNodeIT/testShrinkOnTiers

**Reproduction line:**
`./gradlew ':x-pack:plugin:ilm:internalClusterTest' --tests "org.elasticsearch.xpack.ilm.ILMMultiNodeIT.testShrinkOnTiers" -Dtests.seed=F8EDA3E765E3F9DC -Dtests.locale=fr-LU -Dtests.timezone=Etc/GMT-6 -Druntime.java=11`

**Applicable branches:**
7.17, 8.1

**Reproduces locally?:**
No

**Failure history:**
https://gradle-enterprise.elastic.co/scans/tests?tests.container=org.elasticsearch.xpack.ilm.ILMMultiNodeIT&tests.test=testShrinkOnTiers

**Failure excerpt:**
```
java.lang.AssertionError: Unable to find an ilm explain output for the shrunk index of myindex

  at org.junit.Assert.fail(Assert.java:88)
  at org.junit.Assert.assertTrue(Assert.java:41)
  at org.junit.Assert.assertNotNull(Assert.java:712)
  at org.elasticsearch.xpack.ilm.ILMMultiNodeIT.lambda$testShrinkOnTiers$0(ILMMultiNodeIT.java:154)
  at org.elasticsearch.test.ESTestCase.assertBusy(ESTestCase.java:1142)
  at org.elasticsearch.xpack.ilm.ILMMultiNodeIT.testShrinkOnTiers(ILMMultiNodeIT.java:137)
  at jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(NativeMethodAccessorImpl.java:-2)
  at jdk.internal.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
  at jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
  at java.lang.reflect.Method.invoke(Method.java:566)
  at com.carrotsearch.randomizedtesting.RandomizedRunner.invoke(RandomizedRunner.java:1750)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$8.evaluate(RandomizedRunner.java:938)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$9.evaluate(RandomizedRunner.java:974)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$10.evaluate(RandomizedRunner.java:988)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at org.apache.lucene.util.TestRuleSetupTeardownChained$1.evaluate(TestRuleSetupTeardownChained.java:49)
  at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:45)
  at org.apache.lucene.util.TestRuleThreadAndTestName$1.evaluate(TestRuleThreadAndTestName.java:48)
  at org.apache.lucene.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:64)
  at org.apache.lucene.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:47)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:368)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl.forkTimeoutingTask(ThreadLeakControl.java:817)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl$3.evaluate(ThreadLeakControl.java:468)
  at com.carrotsearch.randomizedtesting.RandomizedRunner.runSingleTest(RandomizedRunner.java:947)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$5.evaluate(RandomizedRunner.java:832)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$6.evaluate(RandomizedRunner.java:883)
  at com.carrotsearch.randomizedtesting.RandomizedRunner$7.evaluate(RandomizedRunner.java:894)
  at org.apache.lucene.util.AbstractBeforeAfterRule$1.evaluate(AbstractBeforeAfterRule.java:45)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at org.apache.lucene.util.TestRuleStoreClassName$1.evaluate(TestRuleStoreClassName.java:41)
  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.util.TestRuleAssertionsRequired$1.evaluate(TestRuleAssertionsRequired.java:53)
  at org.apache.lucene.util.TestRuleMarkFailure$1.evaluate(TestRuleMarkFailure.java:47)
  at org.apache.lucene.util.TestRuleIgnoreAfterMaxFailures$1.evaluate(TestRuleIgnoreAfterMaxFailures.java:64)
  at org.apache.lucene.util.TestRuleIgnoreTestSuites$1.evaluate(TestRuleIgnoreTestSuites.java:54)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl$StatementRunner.run(ThreadLeakControl.java:368)
  at java.lang.Thread.run(Thread.java:829)

```
x-pack/plugin/ilm/src/test/java/org/elasticsearch/xpack/ilm/PolicyStepsRegistryTests.java
.nodes(DiscoveryNodes.builder().localNodeId(nodeId).masterNodeId(nodeId).add(masterNode).build())
                .build();
            registry.update(currentState.metadata().custom(IndexLifecycleMetadata.TYPE));
            assertThat(registeredStepsForPolicy.get(step.getKey()), equalTo(step));
            assertThat(registry.getStep(metadata.index(index), step.getKey()), equalTo(step));
        }
        Map<String, LifecyclePolicyMetadata> registryPolicyMap = registry.getLifecyclePolicyMap();
        Map<String, Step> registryFirstStepMap = registry.getFirstStepMap();
        Map<String, Map<Step.StepKey, Step>> registryStepMap = registry.getStepMap();
        registry.update(currentState.metadata().custom(IndexLifecycleMetadata.TYPE));
        assertThat(registry.getLifecyclePolicyMap(), equalTo(registryPolicyMap));
        assertThat(registry.getFirstStepMap(), equalTo(registryFirstStepMap));
        assertThat(registry.getStepMap(), equalTo(registryStepMap));
        
        lifecycleMetadata = new IndexLifecycleMetadata(Collections.emptyMap(), OperationMode.RUNNING);
        currentState = ClusterState.builder(currentState)
            .metadata(Metadata.builder(metadata).putCustom(IndexLifecycleMetadata.TYPE, lifecycleMetadata))
            .build();
        registry.update(currentState.metadata().custom(IndexLifecycleMetadata.TYPE));
        assertTrue(registry.getLifecyclePolicyMap().isEmpty());
        assertTrue(registry.getFirstStepMap().isEmpty());
        assertTrue(registry.getStepMap().isEmpty());
    }
    public void testUpdateChangedPolicy() {
        Client client = mock(Client.class);
        Mockito.when(client.settings()).thenReturn(Settings.EMPTY);
        String policyName = randomAlphaOfLengthBetween(5, 10);
        LifecyclePolicy newPolicy = LifecyclePolicyTests.randomTestLifecyclePolicy(policyName);
        Map<String, String> headers = new HashMap<>();
        if (randomBoolean()) {