The duplicated LP: 1827119 has fix released with patch: https://review.opendev.org/#/c/677424/ which merged in Sep 23th, 2019.
I have tried with 20191120T023000Z to verify the issue, but cannot reproduce, since set ceph_mon_gib=23 cannot return sucess because the available space in compute-0 have only 1 GB growth.

@Wendy, can you re-check the issue with latest image?

Following is the print information from my evaluation:
[sysadmin@controller-0 ~(keystone_admin)]$ system ceph-mon-list
+--------------------------------------+--------------+--------------+------------+------+
| uuid                                 | ceph_mon_gib | hostname     | state      | task |
+--------------------------------------+--------------+--------------+------------+------+
| 2e78c9e8-e6b8-4f79-b498-92de29538c24 | 20           | controller-0 | configured | None |
| 372658d0-0a44-465a-9d8f-12d6dba6b38e | 20           | controller-1 | configured | None |
| 53a8dce1-e830-4514-97f2-652a9ec5ab9f | 20           | compute-0    | configured | None |
+--------------------------------------+--------------+--------------+---------c

$ system ceph-mon-modify controller-0 ceph_mon_gib=40
Node: compute-0 Total target growth size 20 GiB for database (doubled for upgrades), glance, scratch, backup, extension and ceph-mon exceeds growth limit of 1 GiB.

$ system ceph-mon-modify controller-0 ceph_mon_gib=23
Node: compute-0 Total target growth size 3 GiB for database (doubled for upgrades), glance, scratch, backup, extension and ceph-mon exceeds growth limit of 1 GiB.

$ system ceph-mon-modify controller-0 ceph_mon_gib=21
+--------------------------------------+--------------+--------------+------------+------+
| uuid                                 | ceph_mon_gib | hostname     | state      | task |
+--------------------------------------+--------------+--------------+------------+------+
| 2e78c9e8-e6b8-4f79-b498-92de29538c24 | 21           | controller-0 | configured | None |
| 372658d0-0a44-465a-9d8f-12d6dba6b38e | 21           | controller-1 | configured | None |
| 53a8dce1-e830-4514-97f2-652a9ec5ab9f | 21           | compute-0    | configured | None |
+--------------------------------------+--------------+--------------+------------+------+

NOTE: ceph_mon_gib for both controllers are changed.

System configuration has changed.
please follow the administrator guide to complete configuring system.

# After compute-0 lock and unlock, the system host-list shows normally.

[sysadmin@controller-0 ~(keystone_admin)]$ system host-list
+----+--------------+-------------+----------------+-------------+--------------+
| id | hostname     | personality | administrative | operational | availability |
+----+--------------+-------------+----------------+-------------+--------------+
| 1  | controller-0 | controller  | unlocked       | enabled     | available    |
| 2  | controller-1 | controller  | unlocked       | enabled     | available    |
| 3  | compute-0    | worker      | unlocked       | enabled     | available    |
| 4  | compute-1    | worker      | unlocked       | enabled     | available    |
+----+--------------+-------------+----------------+-------------+--------------+

# ssh to compute-0 and check:
compute-0:~$ lsblk
NAME                         MAJ:MIN RM  SIZE RO TYPE MOUNTPOINT
sda                            8:0    0  200G  0 disk
├─sda1                         8:1    0    1M  0 part
├─sda2                         8:2    0  500M  0 part /boot
├─sda3                         8:3    0   69G  0 part
│ ├─cgts--vg-scratch--lv     253:1    0  3.9G  0 lvm  /scratch
│ ├─cgts--vg-log--lv         253:2    0  3.9G  0 lvm  /var/log
│ ├─cgts--vg-kubelet--lv     253:3    0   10G  0 lvm  /var/lib/kubelet
│ ├─cgts--vg-ceph--mon--lv   253:4    0   21G  0 lvm  /var/lib/ceph/mon
│ └─cgts--vg-docker--lv      253:5    0   30G  0 lvm  /var/lib/docker
├─sda4                         8:4    0 19.5G  0 part /
├─sda5                         8:5    0   10G  0 part
│ └─nova--local-instances_lv 253:0    0   10G  0 lvm  /var/lib/nova/instances
├─sda6                         8:6    0   30G  0 part
├─sda7                         8:7    0   20G  0 part
└─sda8                         8:8    0   20G  0 part
sdb                            8:16   0   30G  0 disk