I found some additional error logs after lowering the Akka log level to DEBUG:
At the beginning of the clustering process, it seems like bootstrapCoordinator runs into this error first:
Probing [http://10-8-2-184.coolapp-testing.pod.cluster.local:8558/bootstrap/seed-nodes] failed due to: Unsupported Content-Type, supported: application/json
2019-02-13T06:12:19.488Z [e[37minfoe[0m] akka.management.AkkaManagement [sourceThread=application-akka.actor.default-dispatcher-2, akkaTimestamp=06:12:19.487UTC, akkaSource=AkkaManagement(akka://application), sourceActorSystem=application] - Bound Akka Management (HTTP) endpoint to: 127.0.0.1:8558
2019-02-13T06:12:20.815Z [e[37minfoe[0m] akka.management.cluster.bootstrap.internal.BootstrapCoordinator [sourceThread=application-akka.actor.default-dispatcher-16, akkaTimestamp=06:12:20.813UTC, akkaSource=akka.tcp://application@10-8-3-223.coolapp-testing.pod.cluster.local:2552/system/bootstrapCoordinator, sourceActorSystem=application] - Located service members based on: [Lookup(admin,Some(management),Some(tcp))]: [ResolvedTarget(10-8-2-184.coolapp-testing.pod.cluster.local,Some(8558),Some(/10.8.2.184)), ResolvedTarget(10-8-1-9.coolapp-testing.pod.cluster.local,Some(8558),Some(/10.8.1.9)), ResolvedTarget(10-8-3-223.coolapp-testing.pod.cluster.local,Some(8558),Some(/10.8.3.223))]
2019-02-13T06:12:20.853Z [e[37minfoe[0m] akka.management.cluster.bootstrap.internal.BootstrapCoordinator [sourceThread=application-akka.actor.default-dispatcher-16, akkaSource=akka.tcp://application@10-8-3-223.coolapp-testing.pod.cluster.local:2552/system/bootstrapCoordinator, sourceActorSystem=application, akkaTimestamp=06:12:20.852UTC] - Located service members based on: [Lookup(admin,Some(management),Some(tcp))]: [ResolvedTarget(10-8-2-184.coolapp-testing.pod.cluster.local,Some(8558),Some(/10.8.2.184)), ResolvedTarget(10-8-1-9.coolapp-testing.pod.cluster.local,Some(8558),Some(/10.8.1.9)), ResolvedTarget(10-8-3-223.coolapp-testing.pod.cluster.local,Some(8558),Some(/10.8.3.223))]
2019-02-13T06:12:21.265Z [e[33mwarne[0m] akka.management.cluster.bootstrap.internal.HttpContactPointBootstrap [sourceThread=application-akka.actor.default-dispatcher-2, akkaTimestamp=06:12:21.257UTC, akkaSource=akka.tcp://application@10-8-3-223.coolapp-testing.pod.cluster.local:2552/system/bootstrapCoordinator/contactPointProbe-10-8-2-184.coolapp-testing.pod.cluster.local-8558, sourceActorSystem=application] - Probing [http://10-8-2-184.coolapp-testing.pod.cluster.local:8558/bootstrap/seed-nodes] failed due to: Unsupported Content-Type, supported: application/json
2019-02-13T06:12:21.266Z [e[37minfoe[0m] akka.management.cluster.bootstrap.internal.BootstrapCoordinator [sourceThread=application-akka.actor.default-dispatcher-16, akkaTimestamp=06:12:21.264UTC, akkaSource=akka.tcp://application@10-8-3-223.coolapp-testing.pod.cluster.local:2552/system/bootstrapCoordinator, sourceActorSystem=application] - Contact point [akka.tcp://application@10-8-1-9.coolapp-testing.pod.cluster.local:2552] returned [1] seed-nodes [akka.tcp://application@10-8-1-9.coolapp-testing.pod.cluster.local:2552]
2019-02-13T06:12:21.320Z [e[37minfoe[0m] akka.management.cluster.bootstrap.internal.BootstrapCoordinator [sourceThread=application-akka.actor.default-dispatcher-16, akkaTimestamp=06:12:21.320UTC, akkaSource=akka.tcp://application@10-8-3-223.coolapp-testing.pod.cluster.local:2552/system/bootstrapCoordinator, sourceActorSystem=application] - Joining [akka.tcp://application@10-8-3-223.coolapp-testing.pod.cluster.local:2552] to existing cluster [akka.tcp://application@10-8-1-9.coolapp-testing.pod.cluster.local:2552]
2019-02-13T06:12:21.717Z [e[37minfoe[0m] akka.management.cluster.bootstrap.contactpoint.HttpClusterBootstrapRoutes [sourceThread=application-akka.actor.default-dispatcher-21, akkaTimestamp=06:12:21.717UTC, akkaSource=HttpClusterBootstrapRoutes(akka://application), sourceActorSystem=application] - Bootstrap request from 127.0.0.1:47636: Contact Point returning 0 seed-nodes ([Set()])