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
)
            .build();
        final var routingNodes = clusterState.mutableRoutingNodes();
        final var unassigned = routingNodes.unassigned().iterator();
        while (unassigned.hasNext()) {
            final var shardRouting = unassigned.next();
            if (shardRouting.primary() && shardRouting.shardId().id() == 1) {
                final var unassignedInfo = shardRouting.unassignedInfo();
                assertThat(unassignedInfo.getLastAllocationStatus(), equalTo(UnassignedInfo.AllocationStatus.NO_ATTEMPT));
                unassigned.updateUnassigned(
                    new UnassignedInfo(
                        unassignedInfo.getReason(),
                        unassignedInfo.getMessage(),
                        unassignedInfo.getFailure(),
                        unassignedInfo.getNumFailedAllocations(),
                        unassignedInfo.getUnassignedTimeInNanos(),
                        unassignedInfo.getUnassignedTimeInMillis(),
                        unassignedInfo.isDelayed(),
                        UnassignedInfo.AllocationStatus.DECIDERS_THROTTLED,
                        unassignedInfo.getFailedNodeIds(),
                        unassignedInfo.getLastAllocatedNodeId()
                    ),
                    shardRouting.recoverySource(),
                    new RoutingChangesObserver.DelegatingRoutingChangesObserver()
                );
            }
        }
        final var routingAllocation = new RoutingAllocation(
            new AllocationDeciders(List.of()),
            routingNodes,