Similar issue was seen when mgmt vlan was brought down on the switch side, all subcloud stayed online, in-sync.
Load:2020-06-11_20-00-00

[sysadmin@controller-0 ~(keystone_admin)]$ dcmanager subcloud list 
+-----+-------------+------------+--------------+---------------+---------+
|  id | name        | management | availability | deploy status | sync    |
+-----+-------------+------------+--------------+---------------+---------+
| 361 | subcloud31  | managed    | online       | complete      | in-sync |
| 362 | subcloud32  | managed    | online       | complete      | in-sync |
| 363 | subcloud33  | managed    | online       | complete      | in-sync |
| 364 | subcloud34  | managed    | online       | complete      | in-sync |
| 367 | subcloud37  | managed    | online       | complete      | in-sync |
| 368 | subcloud38  | managed    | online       | complete      | in-sync |
| 369 | subcloud39  | managed    | online       | complete      | in-sync |
| 370 | subcloud40  | managed    | online       | complete      | in-sync |
| 372 | subcloud42  | managed    | online       | complete      | in-sync |
| 374 | subcloud44  | managed    | online       | complete      | in-sync |
| 376 | subcloud46  | managed    | online       | complete      | in-sync |
| 377 | subcloud47  | managed    | online       | complete      | in-sync |
| 378 | subcloud48  | managed    | online       | complete      | in-sync |
| 431 | subcloud1   | managed    | online       | complete      | in-sync |

[sysadmin@controller-0 ~(keystone_admin)]$ tail -f /var/log/dcmanager/dcmanager.log
2020-06-22 15:56:47.511 281916 ERROR dccommon.drivers.openstack.sdk_platform [-] keystone_client region subcloud16 error: Unable to establish connection to https://[fd01:110::2]:5001/v3/auth/tokens: HTTPSConnectionPool(host='fd01:110::2', port=5001): Max retries exceeded with url: /v3/auth/tokens (Caused by NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7f4af67e1a50>: Failed to establish a new connection: [Errno 113] EHOSTUNREACH',)): ConnectFailure: Unable to establish connection to https://[fd01:110::2]:5001/v3/auth/tokens: HTTPSConnectionPool(host='fd01:110::2', port=5001): Max retries exceeded with url: /v3/auth/tokens (Caused by NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7f4af67e1a50>: Failed to establish a new connection: [Errno 113] EHOSTUNREACH',))
2020-06-22 15:56:47.511 281916 ERROR dcmanager.audit.subcloud_audit_manager [-] Identity or Platform endpoint for online subcloud: subcloud16 not found.: ConnectFailure: Unable to establish connection to https://[fd01:110::2]:5001/v3/auth/tokens: HTTPSConnectionPool(host='fd01:110::2', port=5001): Max retries exceeded with url: /v3/auth/tokens (Caused by NewConnectionError('<urllib3.connection.VerifiedHTTPSConnection object at 0x7f4af67e1a50>: Failed to establish a new connection: [Errno 113] EHOSTUNREACH',))
2020-06-22 15:56:47.515 281503 INFO dcmanager.manager.service [req-c3a5246c-f971-4c09-9ed2-a6ea0a1a10d5 - - - - -] Handling update_subcloud_availability request for: subcloud16
2020-06-22 15:56:47.538 281916 INFO dcmanager.audit.subcloud_audit_manager [-] Notifying dcmanager, subcloud:subcloud16, availability:None