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
public void afterPrimariesBeforeReplicas(RoutingAllocation allocation) {}
        @Override
        public void allocateUnassigned(
            ShardRouting shardRouting,
            RoutingAllocation allocation,
            UnassignedAllocationHandler unassignedAllocationHandler
        ) {}
        @Override
        public AllocateUnassignedDecision explainUnassignedShardAllocation(
            ShardRouting unassignedShard,
            RoutingAllocation routingAllocation
        ) {
            throw new AssertionError("should not be called");
        }
        @Override
        public void cleanCaches() {}
        @Override
        public void applyStartedShards(List<ShardRouting> startedShards, RoutingAllocation allocation) {}
        @Override
        public void applyFailedShards(List<FailedShard> failedShards, RoutingAllocation allocation) {}
        @Override
        public int getNumberOfInFlightFetches() {
            return 0;
        }
    }
    private static DesiredBalance desiredBalance(ClusterState clusterState, BiPredicate<ShardId, String> isDesiredPredicate) {
        return new DesiredBalance(
            1,
            StreamSupport.stream(clusterState.routingTable().spliterator(), false)
                .flatMap(indexRoutingTable -> IntStream.range(0, indexRoutingTable.size()).mapToObj(indexRoutingTable::shard))