Ceph pool for ext4 is not created

The ceph pool for ext4 isn't created properly.

$ juju run-action ceph-mon/0 list-pools --wait
unit-ceph-mon-0:
  id: df04d049-0466-4d7b-837a-607da713cbfd
  results:
    message: |
      0 rbd,1 xfs-pool,
  status: completed
  timing:
    completed: 2019-04-09 22:41:32 +0000 UTC
    enqueued: 2019-04-09 22:41:30 +0000 UTC
    started: 2019-04-09 22:41:31 +0000 UTC
  unit: ceph-mon/0

The logs show:

ceph-storage:17: Request already sent but not complete, not sending new request

more logs:

unit-kubernetes-master-1: 18:03:04 INFO unit.kubernetes-master/1.juju-log ceph-storage:17: Creating Ceph pools.
unit-kubernetes-master-1: 18:03:05 INFO unit.kubernetes-master/1.juju-log ceph-storage:17: status-set: maintenance: Creating xfs-pool pool.
unit-kubernetes-master-0: 18:03:05 DEBUG unit.kubernetes-master/0.juju-log ceph-storage:17: tracer: set flag config.set.dns-provider
unit-kubernetes-master-0: 18:03:05 DEBUG unit.kubernetes-master/0.juju-log ceph-storage:17: tracer: set flag config.default.dns-provider
unit-kubernetes-master-1: 18:03:05 DEBUG unit.kubernetes-master/1.juju-log ceph-storage:17: Sending request 403268bc-5b13-11e9-adea-12aebd2512ca
unit-kubernetes-master-0: 18:03:05 DEBUG unit.kubernetes-master/0.juju-log ceph-storage:17: tracer: set flag config.set.dns_domain
unit-kubernetes-master-1: 18:03:05 INFO unit.kubernetes-master/1.juju-log ceph-storage:17: status-set: maintenance: Creating ext4-pool pool.
unit-kubernetes-master-0: 18:03:05 DEBUG unit.kubernetes-master/0.juju-log ceph-storage:17: tracer: set flag config.default.dns_domain
unit-kubernetes-master-1: 18:03:05 INFO unit.kubernetes-master/1.juju-log ceph-storage:17: Json request: {"api-version": 1, "ops": [{"op": "create-pool", "name": "xfs-pool", "replicas": 3, "pg_num": null, "weight": null, "group": null, "group-namespace": null, "app-name": null, "max-bytes": null, "max-objects": null}], "request-id": "403268bc-5b13-11e9-adea-12aebd2512ca"}
unit-kubernetes-master-0: 18:03:05 DEBUG unit.kubernetes-master/0.juju-log ceph-storage:17: tracer: set flag config.default.extra_sans
unit-kubernetes-master-1: 18:03:05 DEBUG unit.kubernetes-master/1.juju-log ceph-storage:17: Request already sent but not complete, not sending new request
unit-kubernetes-master-0: 18:03:05 DEBUG unit.kubernetes-master/0.juju-log ceph-storage:17: tracer: set flag config.set.service-cidr
unit-kubernetes-master-1: 18:03:05 DEBUG unit.kubernetes-master/1.juju-log ceph-storage:17: tracer>
tracer: set flag kubernetes-master.ceph.pool.created
tracer: -- dequeue handler reactive/kubernetes_master.py:1074:ceph_storage_pool