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
public Decision canRebalance(RoutingAllocation allocation) {
                    return Decision.NO;
                }
                @Override
                public Decision canAllocate(ShardRouting shardRouting, RoutingAllocation allocation) {
                    return canAllocateRef.get();
                }
            }
        );
        boolean changed;
        do {
            final var newState = startInitializingShardsAndReroute(allocationService, clusterState);
            changed = newState != clusterState;
            clusterState = newState;
        } while (changed);
        for (final var shardRouting : clusterState.routingTable().allShards()) {
            assertTrue(shardRouting.started());
            assertThat(shardRouting.currentNodeId(), oneOf("node-0", "node-1"));
        }
        clusterSettings.applySettings(
            Settings.builder()
                .putList(
                    FilterAllocationDecider.CLUSTER_ROUTING_INCLUDE_GROUP_SETTING.getConcreteSettingForNamespace("_id").getKey(),
                    "node-2",
                    "node-3"
                )
                .build()
        );
        assertSame(clusterState, allocationService.reroute(clusterState, "test", ActionListener.noop()));