I've run a lab with following OpenStack bundle: https://pastebin.canonical.com/p/4jm8DmSPj4/

Counting with 2 Cinder AZs, both connected to Ceph (to make sure it will be Up):
$ cinder service-list
+------------------+-------------------------------+------+---------+-------+----------------------------+-----------------+
| Binary           | Host                          | Zone | Status  | State | Updated_at                 | Disabled Reason |
+------------------+-------------------------------+------+---------+-------+----------------------------+-----------------+
| cinder-scheduler | cinder-az1                    | az1  | enabled | up    | 2019-09-16T12:15:22.000000 | -               |
| cinder-scheduler | juju-d23cf7-1-lxd-1           | az1  | enabled | down  | 2019-09-14T15:29:36.000000 | -               |
| cinder-volume    | cinder-az1@cinder-ceph        | az1  | enabled | up    | 2019-09-16T12:15:22.000000 | -               |
| cinder-volume    | cinder-volume-az2@cinder-ceph | az2  | enabled | up    | 2019-09-16T12:15:27.000000 | -               |
| cinder-volume    | juju-d23cf7-1-lxd-1@LVM       | az1  | enabled | down  | 2019-09-14T15:29:28.000000 | -               |
| cinder-volume    | juju-d23cf7-1-lxd-2@LVM       | az2  | enabled | down  | 2019-09-14T15:28:59.000000 | -               |
+------------------+-------------------------------+------+---------+-------+----------------------------+-----------------+

I've booted 7 cs:ubuntu VMs on this OpenStack with following bundle: https://pastebin.canonical.com/p/VnSPZY4FSm/

Notice that I am using root-disk=volume constraint.

I can see that all volumes map to "az1":

$ for i in $(openstack volume list | tail -n +4 | head -n -1 | awk '{print $2}'); do echo $i; openstack volume show $i | grep "availability_zone"; done
f69c2274-134c-4cab-b948-276b84aa18f2
| availability_zone              | az1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
4a099ccc-e150-4b42-9622-0200e68e442a
| availability_zone              | az1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
84504bc3-75f6-4fa4-a547-03e8e7807e51
| availability_zone              | az1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
8717692b-ec30-41ac-8289-dc706df1b48e
| availability_zone              | az1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
beedf30c-1125-4c10-87c4-d1077c0b21e6
| availability_zone              | az1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
7a9f8f75-e4c0-4baf-b39f-f57b274e5ead
| availability_zone              | az1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |
258563e3-16aa-4576-9cc3-8b39aa826c8d
| availability_zone              | az1                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                               |


We can see that out-of-the-box OpenStack deployment will have similar whenever using root-disk=volume. 

Also, to add AZs to Cinder, one needs to use config-flags to add to standard cinder charm, such as:
  cinder-az1:
    annotations:
      gui-x: '750'
      gui-y: '0'
    charm: cs:cinder
    num_units: 1
    options:
      block-device: None
      glance-api-version: 2
      worker-multiplier: *worker-multiplier
      openstack-origin: *openstack-origin
      config-flags: storage_availability_zone=az1,default_storage_availability_zone=az1
    to:
    - 'lxd:1'
  cinder-volume-az2:
    annotations:
      gui-x: '750'
      gui-y: '0'
    charm: cs:cinder
    num_units: 1
    options:
      enabled-services: volume
      block-device: None
      glance-api-version: 2
      worker-multiplier: *worker-multiplier
      openstack-origin: *openstack-origin
      config-flags: "storage_availability_zone=az2" 
    to:
    - lxd:1