I've seen three builds fail now witht he same error and this started happening on Nov 7 so it's likely we've introduced something here:

https://gradle-enterprise.elastic.co/scans/failures?failures.failureClassification=all_failures&failures.failureMessage=Execution%20failed%20for%20task%20%27:x-pack:qa:rolling-upgrade-multi-cluster:v8.6.0%23follower%23oneThirdUpgradedTest%27.%0A%3E%20%60cluster%7B:x-pack:qa:rolling-upgrade-multi-cluster:v8.6.0-follower%7D%60%20failed%20to%20wait%20for%20cluster%20health%20yellow%20after%2040%20SECONDS%0A%20%20%20%20IO%20error%20while%20waiting%20cluster%0A%20%20%20%20%20%20503%20Service%20Unavailable&search.relativeStartTime=P28D&search.timeZoneId=America/Los_Angeles#

```
* What went wrong:
Execution failed for task ':x-pack:qa:rolling-upgrade-multi-cluster:v8.6.0#follower#oneThirdUpgradedTest'.
> `cluster{:x-pack:qa:rolling-upgrade-multi-cluster:v8.6.0-follower}` failed to wait for cluster health yellow after 40 SECONDS
    IO error while waiting cluster
      503 Service Unavailable
```

It seems the follower cluster is having issues coming back up after upgrade.
server/src/test/java/org/elasticsearch/cluster/routing/allocation/allocator/DesiredBalanceReconcilerTests.java
.stream()
                .allMatch(
                    shardRouting -> shardRouting.unassignedInfo().getLastAllocationStatus() == UnassignedInfo.AllocationStatus.NO_ATTEMPT
                )
        );
    }
    public void testUnassignedSetsAllocationStatusOnUnassignedShards() {
        final var discoveryNodes = discoveryNodes(2);
        final var metadata = Metadata.builder();
        final var routingTable = RoutingTable.builder();
        final var indexMetadata = randomPriorityIndex("index-0", 1, between(0, 5));
        metadata.put(indexMetadata, true);
        routingTable.addAsNew(indexMetadata);
        final var clusterState = ClusterState.builder(ClusterName.DEFAULT)
            .nodes(discoveryNodes)
            .metadata(metadata)
            .routingTable(routingTable)
            .build();
        final var settings = Settings.EMPTY;
        final var clusterSettings = new ClusterSettings(settings, ClusterSettings.BUILT_IN_CLUSTER_SETTINGS);
        final var assignPrimary = new AtomicBoolean(false);
        final var nonYesDecision = randomFrom(Decision.THROTTLE, Decision.NO);
        final var desiredBalance = desiredBalance(clusterState, (shardId, nodeId) -> true);
        final var allocationService = createTestAllocationService(
            routingAllocation -> reconcile(routingAllocation, desiredBalance),
            new SameShardAllocationDecider(settings, clusterSettings),
            new ReplicaAfterPrimaryActiveAllocationDecider(),
            new AllocationDecider() {
                @Override
                public Decision canAllocate(ShardRouting shardRouting, RoutingNode node, RoutingAllocation allocation) {