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
UnassignedInfo.AllocationStatus.DECIDERS_NO,
                        unassignedInfo.getFailedNodeIds(),
                        unassignedInfo.getLastAllocatedNodeId()
                    ),
                    shardRouting.recoverySource(),
                    allocation.changes()
                );
            }
        }
    }
    private void allocateUnassigned() {
        RoutingNodes.UnassignedShards unassigned = routingNodes.unassigned();
        if (logger.isTraceEnabled()) {
            logger.trace("Start allocating unassigned shards");
        }
        if (unassigned.isEmpty()) {
            return;
        }
        /*
         * TODO: We could be smarter here and group the shards by index and then
         * use the sorter to save some iterations.
         */
        final PriorityComparator secondaryComparator = PriorityComparator.getAllocationComparator(allocation);
        final Comparator<ShardRouting> comparator = (o1, o2) -> {
            if (o1.primary() ^ o2.primary()) {
                return o1.primary() ? -1 : 1;
            }
            if (o1.getIndexName().compareTo(o2.getIndexName()) == 0) {
                return o1.getId() - o2.getId();
            }