A few questions from looking at the logs:

* Please could you provide approximate times when each step was taken (charm upgrade, upgrade to train, upgrade to ussuri).
* Was the upgrade done via action managed upgrades ?
* Are the units still going into a masked state on config-changed events? If so the db flags would be very useful.

fwiw looking at the logs all I can see is the units initially masking their services waiting for the leader to signal that the db sync is done and then the services being unmasked when the signal arrives:

nova-cloud-controller/2

Unit starts
2021-06-07 23:49:45 INFO juju unit_agent.go:253 Starting unit workers for "nova-cloud-controller/2"

2021-06-07 23:52:59 INFO juju-log Disabling services into db relation joined

2021-06-08 00:04:58 INFO juju-log Loaded template from templates/train/nova.conf
2021-06-08 00:05:00 INFO juju-log Database sync not ready. Shutting down services
2021-06-08 00:10:50 INFO juju-log cluster:33: Database sync not ready. Shutting down services
2021-06-08 00:12:01 INFO juju-log cluster:33: Database sync not ready. Shutting down services

nova-cloud-controller/0
2021-06-08 00:14:43 INFO juju-log shared-db:189: Informing peers that dbsync is complete



nova-cloud-controller/2
2021-06-08 00:14:44 DEBUG jujuc server.go:211 running hook tool "juju-log" for nova-cloud-controller/2-leader-settings-changed-1071374215362069442
2021-06-08 00:14:49 WARNING unit.nova-cloud-controller/2.leader-settings-changed logger.go:60 Executing: /lib/systemd/systemd-sysv-install enable apache2
2021-06-08 00:14:50 WARNING unit.nova-cloud-controller/2.leader-settings-changed logger.go:60 Executing: /lib/systemd/systemd-sysv-install enable memcached
2021-06-08 00:14:51 WARNING unit.nova-cloud-controller/2.leader-settings-changed logger.go:60 Executing: /lib/systemd/systemd-sysv-install enable nova-scheduler
2021-06-08 00:14:52 WARNING unit.nova-cloud-controller/2.leader-settings-changed logger.go:60 Executing: /lib/systemd/systemd-sysv-install enable nova-spiceproxy
2021-06-08 00:14:54 WARNING unit.nova-cloud-controller/2.leader-settings-changed logger.go:60 Executing: /lib/systemd/systemd-sysv-install enable haproxy
2021-06-08 00:14:55 WARNING unit.nova-cloud-controller/2.leader-settings-changed logger.go:60 Executing: /lib/systemd/systemd-sysv-install enable nova-conductor
2021-06-08 00:15:08 WARNING unit.nova-cloud-controller/2.leader-settings-changed logger.go:60 Executing: /lib/systemd/systemd-sysv-install enable nova-scheduler
2021-06-08 00:15:09 WARNING unit.nova-cloud-controller/2.leader-settings-changed logger.go:60 Executing: /lib/systemd/systemd-sysv-install enable nova-spiceproxy
2021-06-08 00:15:10 WARNING unit.nova-cloud-controller/2.leader-settings-changed logger.go:60 Executing: /lib/systemd/systemd-sysv-install enable memcached
2021-06-08 00:15:12 WARNING unit.nova-cloud-controller/2.leader-settings-changed logger.go:60 Executing: /lib/systemd/systemd-sysv-install enable nova-conductor
2021-06-08 00:15:13 WARNING unit.nova-cloud-controller/2.leader-settings-changed logger.go:60 Executing: /lib/systemd/systemd-sysv-install enable haproxy
2021-06-08 00:15:14 WARNING unit.nova-cloud-controller/2.leader-settings-changed logger.go:60 Executing: /lib/systemd/systemd-sysv-install enable apache2