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
if (shardRouting.primary()) {
                        return assignPrimary.get() ? Decision.YES : nonYesDecision;
                    } else {
                        return nonYesDecision;
                    }
                }
            }
        );
        final var redState = startInitializingShardsAndReroute(allocationService, clusterState);
        assertEquals(
            nonYesDecision == Decision.NO
                ? UnassignedInfo.AllocationStatus.DECIDERS_NO
                : UnassignedInfo.AllocationStatus.DECIDERS_THROTTLED,
            redState.routingTable().shardRoutingTable("index-0", 0).primaryShard().unassignedInfo().getLastAllocationStatus()
        );
        assignPrimary.set(true);
        final var yellowState = startInitializingShardsAndReroute(
            allocationService,
            startInitializingShardsAndReroute(allocationService, redState)
        );
        for (final var shardRouting : yellowState.routingTable().shardRoutingTable("index-0", 0).replicaShards()) {
            assertEquals(UnassignedInfo.AllocationStatus.NO_ATTEMPT, shardRouting.unassignedInfo().getLastAllocationStatus());
        }
    }
    public void testUnassignedPrimariesThrottlingAndFallback() {
        
        
        final var discoveryNodes = discoveryNodes(2);
        final var metadata = Metadata.builder();
        final var routingTable = RoutingTable.builder();