Looks like a BWC problem related to the multiple deployments work.

**Build scan:**
https://gradle-enterprise.elastic.co/s/kckoxldn5uysg/tests/:x-pack:qa:rolling-upgrade:v8.0.1%23upgradedClusterTest/org.elasticsearch.upgrades.MLModelDeploymentsUpgradeIT/testTrainedModelDeploymentStopOnMixedCluster

**Reproduction line:**
```
./gradlew ':x-pack:qa:rolling-upgrade:v8.0.1#upgradedClusterTest' -Dtests.class="org.elasticsearch.upgrades.MLModelDeploymentsUpgradeIT" -Dtests.method="testTrainedModelDeploymentStopOnMixedCluster" -Dtests.seed=7AF5FD52AC644351 -Dtests.bwc=true -Dtests.locale=zh-TW -Dtests.timezone=Europe/Vaduz -Druntime.java=20
```

**Applicable branches:**
main

**Reproduces locally?:**
Didn't try

**Failure history:**
https://gradle-enterprise.elastic.co/scans/tests?tests.container=org.elasticsearch.upgrades.MLModelDeploymentsUpgradeIT&tests.test=testTrainedModelDeploymentStopOnMixedCluster

**Failure excerpt:**
```
java.lang.AssertionError: 
Expected: "{}"
     but: was "{\"metadata\":{\"trained_model_allocation\":{}}}"

  at __randomizedtesting.SeedInfo.seed([7AF5FD52AC644351:611D2326BF7A84A8]:0)
  at org.hamcrest.MatcherAssert.assertThat(MatcherAssert.java:18)
  at org.junit.Assert.assertThat(Assert.java:956)
  at org.junit.Assert.assertThat(Assert.java:923)
  at org.elasticsearch.upgrades.MLModelDeploymentsUpgradeIT.assertThatTrainedModelAssignmentMetadataIsEmpty(MLModelDeploymentsUpgradeIT.java:269)
  at org.elasticsearch.upgrades.MLModelDeploymentsUpgradeIT.testTrainedModelDeploymentStopOnMixedCluster(MLModelDeploymentsUpgradeIT.java:158)
  at jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
  at java.lang.reflect.Method.invoke(Method.java:578)
  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: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$1.evaluate(TestRuleMarkFailure.java:44)
  at com.carrotsearch.randomizedtesting.rules.StatementAdapter.evaluate(StatementAdapter.java:36)
  at com.carrotsearch.randomizedtesting.ThreadLeakControl$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:1623)

```
x-pack/qa/rolling-upgrade/src/test/java/org/elasticsearch/upgrades/MLModelDeploymentsUpgradeIT.java
@AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/95360")
    public void testTrainedModelDeployment() throws Exception {
        assumeTrue("NLP model deployments added in 8.0", UPGRADE_FROM_VERSION.onOrAfter(Version.V_8_0_0));
        final String modelId = "upgrade-deployment-test";
        switch (CLUSTER_TYPE) {
            case OLD -> {
                setupDeployment(modelId);
                assertInfer(modelId);
            }
            case MIXED -> {
                ensureHealth(".ml-inference-*,.ml-config*", (request -> {
                    request.addParameter("wait_for_status", "yellow");
                    request.addParameter("timeout", "70s");
                }));
                waitForDeploymentStarted(modelId);
                
                for (int i = 0; i < 10; i++) {
                    assertInfer(modelId);
                }
            }
            case UPGRADED -> {
                ensureHealth(".ml-inference-*,.ml-config*", (request -> {
                    request.addParameter("wait_for_status", "yellow");
                    request.addParameter("timeout", "70s");
                }));
                waitForDeploymentStarted(modelId);
                assertInfer(modelId);
                assertNewInfer(modelId);
                stopDeployment(modelId);
            }