I got this too.  #6 works in my case too after reboot and resolved for ceph-mon/0.  The ceph cluster is still working and operation. 

zlmao@p14s:~/gzgz/upgrade$ juju status ceph-mon
Model      Controller  Cloud/Region  Version  SLA          Timestamp
openstack  maas        maas/default  2.9.45   unsupported  16:21:09+08:00

App       Version  Status   Scale  Charm     Channel         Rev  Exposed  Message
ceph-mon  15.2.17  waiting      3  ceph-mon  pacific/stable  189  no       Waiting on juju-e5b537-0-lxd-0 to finish upgrading

Unit         Workload     Agent      Machine  Public address  Ports  Message
ceph-mon/0   maintenance  executing  0/lxd/0  192.168.222.69         (config-changed) Finishing upgrade
ceph-mon/1*  waiting      executing  1/lxd/0  192.168.222.65         (config-changed) Waiting on juju-e5b537-0-lxd-0 to finish upgrading
ceph-mon/2   waiting      executing  2/lxd/0  192.168.222.96         (config-changed) Waiting on juju-e5b537-1-lxd-0 to finish upgrading

root@juju-e5b537-0-lxd-0:~# tail -n10 /var/log/juju/unit-ceph-mon-0.log 
2024-01-04 07:39:21 INFO unit.ceph-mon/0.juju-log server.go:316 Upgrading to: pacific
2024-01-04 07:39:21 WARNING unit.ceph-mon/0.config-changed logger.go:60 set mon_juju-e5b537-0-lxd-0_pacific_alive
2024-01-04 07:39:21 INFO unit.ceph-mon/0.juju-log server.go:316 Installing [] with options: ['--option=Dpkg::Options::=--force-confold']
2024-01-04 07:39:24 INFO unit.ceph-mon/0.juju-log server.go:316 Installing ['ceph', 'gdisk', 'radosgw', 'xfsprogs', 'lvm2', 'parted', 'smartmontools', 'btrfs-progs'] with options: ['--option=Dpkg::Options::=--force-confold']
2024-01-04 07:39:25 INFO unit.ceph-mon/0.juju-log server.go:316 restarting ceph-mgr.target maybe: True
2024-01-04 07:39:25 INFO unit.ceph-mon/0.juju-log server.go:316 Making dir /var/lib/ceph/mon/ceph-juju-e5b537-0-lxd-0 ceph:ceph 755
2024-01-04 07:39:25 INFO unit.ceph-mon/0.juju-log server.go:316 starting ceph-mgr.target maybe: True
2024-01-04 07:39:25 INFO unit.ceph-mon/0.juju-log server.go:316 Done
2024-01-04 07:39:25 INFO unit.ceph-mon/0.juju-log server.go:316 monitor_key_set mon_juju-e5b537-0-lxd-0_pacific_done 1704353965.4807012
2024-01-04 07:39:25 WARNING unit.ceph-mon/0.config-changed logger.go:60 set mon_juju-e5b537-0-lxd-0_pacific_done
root@juju-e5b537-0-lxd-0:~# 

ubuntu@juju-e5b537-1-lxd-0:~$ sudo tail -n10 /var/log/juju/unit-ceph-mon-1.log 
2024-01-04 07:38:29 INFO unit.ceph-mon/1.juju-log server.go:316 roll_monitor_cluster called with pacific
2024-01-04 07:38:29 INFO unit.ceph-mon/1.juju-log server.go:316 monitor_list: ['juju-e5b537-1-lxd-0', 'juju-e5b537-2-lxd-0', 'juju-e5b537-0-lxd-0']
2024-01-04 07:38:29 WARNING unit.ceph-mon/1.juju-log server.go:316 DEPRECATION WARNING: Function one_shot_log is being removed : Support for use of upstream ``apt_pkg`` module in conjunctionwith charm-helpers is deprecated since 2019-06-25
2024-01-04 07:38:29 INFO unit.ceph-mon/1.juju-log server.go:316 Current Ceph version is 15.2
2024-01-04 07:38:29 INFO unit.ceph-mon/1.juju-log server.go:316 Upgrading to: pacific
2024-01-04 07:38:30 INFO unit.ceph-mon/1.juju-log server.go:316 Installing ['ubuntu-cloud-keyring'] with options: ['--option=Dpkg::Options::=--force-confold']
2024-01-04 07:38:35 INFO unit.ceph-mon/1.juju-log server.go:316 Installing ['ceph', 'gdisk', 'radosgw', 'xfsprogs', 'lvm2', 'parted', 'smartmontools', 'btrfs-progs'] with options: ['--option=Dpkg::Options::=--force-confold']
2024-01-04 07:39:31 INFO unit.ceph-mon/1.juju-log server.go:316 Packages upgraded but not restarting daemons yet.
2024-01-04 07:39:31 INFO unit.ceph-mon/1.juju-log server.go:316 upgrade position: 1
2024-01-04 07:39:32 INFO unit.ceph-mon/1.juju-log server.go:316 Previous node is: juju-e5b537-0-lxd-0
ubuntu@juju-e5b537-1-lxd-0:~$ 

ubuntu@juju-e5b537-2-lxd-0:~$ sudo tail -n10 /var/log/juju/unit-ceph-mon-2.log 
2024-01-04 07:38:29 INFO unit.ceph-mon/2.juju-log server.go:316 Upgrading to: pacific
2024-01-04 07:38:30 INFO unit.ceph-mon/2.juju-log server.go:316 Installing ['ubuntu-cloud-keyring'] with options: ['--option=Dpkg::Options::=--force-confold']
2024-01-04 07:38:35 INFO unit.ceph-mon/2.juju-log server.go:316 Installing ['ceph', 'gdisk', 'radosgw', 'xfsprogs', 'lvm2', 'parted', 'smartmontools', 'btrfs-progs'] with options: ['--option=Dpkg::Options::=--force-confold']
2024-01-04 07:39:08 INFO unit.ceph-mon/2.juju-log server.go:316 Packages upgraded but not restarting daemons yet.
2024-01-04 07:39:08 INFO unit.ceph-mon/2.juju-log server.go:316 upgrade position: 2
2024-01-04 07:39:08 INFO unit.ceph-mon/2.juju-log server.go:316 Previous node is: juju-e5b537-1-lxd-0
2024-01-04 07:39:08 WARNING unit.ceph-mon/2.config-changed logger.go:60 Error ENOENT: key 'mon_juju-e5b537-1-lxd-0_pacific_start' doesn't exist
2024-01-04 07:39:08 INFO unit.ceph-mon/2.juju-log server.go:316 wait_until: waiting for 8 seconds
2024-01-04 07:39:17 WARNING unit.ceph-mon/2.config-changed logger.go:60 Error ENOENT: key 'mon_juju-e5b537-1-lxd-0_pacific_start' doesn't exist
2024-01-04 07:39:17 INFO unit.ceph-mon/2.juju-log server.go:316 wait_until: waiting for 19 seconds
ubuntu@juju-e5b537-2-lxd-0:~$