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/main/java/org/elasticsearch/cluster/routing/allocation/allocator/DesiredBalanceReconciler.java
return;
        }
        if (desiredBalance.assignments().isEmpty()) {
            
            logger.trace("desired balance is empty, nothing to reconcile");
            return;
        }
        
        
        logger.trace("Reconciler#allocateUnassigned");
        allocateUnassigned();
        assert allocateUnassignedInvariant();
        
        logger.trace("Reconciler#moveShards");
        moveShards();
        
        logger.trace("Reconciler#balance");
        balance();
        logger.debug("Reconciliation is complete");
    }
    private boolean allocateUnassignedInvariant() {
        
        assert routingNodes.unassigned().isEmpty();
        final var shardCounts = allocation.metadata()
            .stream()
            .flatMap(
                indexMetadata -> IntStream.range(0, indexMetadata.getNumberOfShards())
                    .mapToObj(
                        shardId -> Tuple.tuple(new ShardId(indexMetadata.getIndex(), shardId), indexMetadata.getNumberOfReplicas() + 1)
                    )