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/DesiredBalanceShardsAllocatorTests.java
public void testGatewayAllocatorPreemptsAllocation() {
        testAllocate(
            (allocation, unassignedAllocationHandler) -> unassignedAllocationHandler.initialize(
                allocation.nodes().getLocalNodeId(),
                null,
                0L,
                allocation.changes()
            ),
            routingTable -> assertTrue(routingTable.index("test-index").shard(0).primaryShard().assignedToNode())
        );
    }
    public void testGatewayAllocatorStillFetching() {
        testAllocate(
            (allocation, unassignedAllocationHandler) -> unassignedAllocationHandler.removeAndIgnore(
                UnassignedInfo.AllocationStatus.FETCHING_SHARD_DATA,
                allocation.changes()
            ),
            routingTable -> {
                var shardRouting = routingTable.shardRoutingTable("test-index", 0).primaryShard();
                assertFalse(shardRouting.assignedToNode());
                assertThat(
                    shardRouting.unassignedInfo().getLastAllocationStatus(),
                    equalTo(UnassignedInfo.AllocationStatus.FETCHING_SHARD_DATA)
                );
            }
        );