Example: 
On CEPH Monitor juju-machine-3-lxc-0 with the following IP's: 
IP address for eth0: 10.198.244.29 
IP address for eth1: 192.168.100.80 [CLUSTER] 
IP address for eth2: 192.168.100.167 [PUBLIC] 

Initial State: 
ceph.conf [mon host]: 
mon host = 
192.168.100.155:6789 
192.168.100.167:6789 
192.168.100.247:6789 

After Reboot: 
ceph.conf [mon host]: 
mon host = 
10.198.244.29:6789 
192.168.100.155:6789 
192.168.100.247:6789 

Observation: 
Notice that after the reboot the [192.168.100.167:6789] IP was replaced by [10.198.244.29:6789]. 
This is not only an incorrect configuration but also an undesirable behavior.