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
private void moveShards() {
        
        
        
        for (final var iterator = routingNodes.nodeInterleavedShardIterator(); iterator.hasNext();) {
            final var shardRouting = iterator.next();
            if (shardRouting.started() == false) {
                
                continue;
            }
            final var assignment = desiredBalance.getAssignment(shardRouting.shardId());
            if (assignment == null) {
                
                continue;
            }
            if (assignment.nodeIds().contains(shardRouting.currentNodeId())) {
                
                continue;
            }
            if (allocation.deciders().canAllocate(shardRouting, allocation).type() != Decision.Type.YES) {
                
                continue;
            }
            final var routingNode = routingNodes.node(shardRouting.currentNodeId());
            final var canRemainDecision = allocation.deciders().canRemain(shardRouting, routingNode, allocation);
            if (canRemainDecision.type() != Decision.Type.NO) {
                
                
                continue;
            }