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
continue;
            }
            if (allocation.deciders().canRebalance(shardRouting, allocation).type() != Decision.Type.YES) {
                
                continue;
            }
            if (allocation.deciders().canAllocate(shardRouting, allocation).type() != Decision.Type.YES) {
                
                continue;
            }
            final var rebalanceTarget = findRelocationTarget(shardRouting, assignment.nodeIds(), this::decideCanAllocate);
            if (rebalanceTarget != null) {
                routingNodes.relocateShard(
                    shardRouting,
                    rebalanceTarget.getId(),
                    allocation.clusterInfo().getShardSize(shardRouting, ShardRouting.UNAVAILABLE_EXPECTED_SHARD_SIZE),
                    allocation.changes()
                );
            }
        }
    }
    private DiscoveryNode findRelocationTarget(final ShardRouting shardRouting, Set<String> desiredNodeIds) {
        final var moveDecision = findRelocationTarget(shardRouting, desiredNodeIds, this::decideCanAllocate);
        if (moveDecision != null) {
            return moveDecision;
        }
        final var shutdown = allocation.metadata().nodeShutdowns().get(shardRouting.currentNodeId());
        final var shardsOnReplacedNode = shutdown != null && shutdown.getType().equals(SingleNodeShutdownMetadata.Type.REPLACE);
        if (shardsOnReplacedNode) {