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
nodeIds.add(shardRouting.currentNodeId());
                    } else {
                        assertTrue(shardRouting.unassigned());
                    }
                }
                assertTrue(desiredBalance.getAssignment(indexShardRoutingTable.shardId()).nodeIds().containsAll(nodeIds));
            }
        }
        assertNotEquals(anyAssigned, desiredBalance.assignments().values().stream().map(ShardAssignment::nodeIds).allMatch(Set::isEmpty));
    }
    public void testUnassignedAllocationPredictsDiskUsage() {
        final var discoveryNodes = discoveryNodes(1);
        final var metadata = Metadata.builder();
        final var routingTable = RoutingTable.builder();
        final var existingIndexMetadata = randomPriorityIndex("index-existing", 1, 0);
        metadata.put(existingIndexMetadata, true);
        routingTable.addAsRecovery(existingIndexMetadata);
        final var restoredIndexMetadata = randomPriorityIndex("index-restored", 1, 0);
        metadata.put(restoredIndexMetadata, true);
        final var recoverySource = new RecoverySource.SnapshotRecoverySource(
            UUIDs.randomBase64UUID(random()),
            new Snapshot("repo", new SnapshotId("snap", UUIDs.randomBase64UUID(random()))),
            Version.CURRENT,
            new IndexId("index", UUIDs.randomBase64UUID(random()))
        );
        routingTable.addAsRestore(restoredIndexMetadata, recoverySource);
        final var clusterState = ClusterState.builder(ClusterName.DEFAULT)
            .nodes(discoveryNodes)
            .metadata(metadata)
            .routingTable(routingTable)