Run into this problem with a customer during a POC setup. I duplicated problem on my system starting with 1um 1pm. Then did an addModule UM2 and it configured UM2 with unassigned as the host-name and ip-address when I enter 'y' in the third argument, which is new for 1.2.5.

Entering the Ip address as the hostname is valid. The new code assumed a host-name would be entered, I assume.

mcsadmin> getsystemn
getsystemnetworkconfig Fri Oct 11 15:55:37 2019

System Network Configuration

Module Name Module Description NIC ID Host Name IP Address
----------- ------------------------- ------ ------------------------------------------- ---------------
um1 User Module #1 1 172.31.43.139 172.31.43.139
pm1 Performance Module #1 1 ip-172-31-34-172.us-west-2.compute.internal 172.31.34.172

mcsadmin> addmodule um2 y 172.31.43.54 ssh
addmodule Fri Oct 11 15:56:04 2019

Adding Modules um2, please wait...
Add Module(s) successfully completed
addModule Command Successfully completed: Modules are Disabled, run alterSystem-enableModule command to enable them

mcsadmin> getsystemn
getsystemnetworkconfig Fri Oct 11 15:58:39 2019

System Network Configuration

Module Name Module Description NIC ID Host Name IP Address
----------- ------------------------- ------ ------------------------------------------- ---------------
um1 User Module #1 1 172.31.43.139 172.31.43.139
um2 User Module #2 1 unassigned unassigned
pm1 Performance Module #1 1 ip-172-31-34-172.us-west-2.compute.internal 172.31.34.172

removed module and reran with the argument as n

mcsadmin> addmodule um2 n 172.31.43.54 ssh
addmodule Fri Oct 11 16:01:07 2019

Adding Modules um2, please wait...
Add Module(s) successfully completed
addModule Command Successfully completed: Modules are Disabled, run alterSystem-enableModule command to enable them

mcsadmin> getsystemn
getsystemnetworkconfig Fri Oct 11 16:03:34 2019

System Network Configuration

Module Name Module Description NIC ID Host Name IP Address
----------- ------------------------- ------ ------------------------------------------- ---------------
um1 User Module #1 1 172.31.43.139 172.31.43.139
um2 User Module #2 1 172.31.43.54 172.31.43.54
pm1 Performance Module #1 1 ip-172-31-34-172.us-west-2.compute.internal 172.31.34.172

So it looks like extra code is needed. If user enters 'y' and ip-address for host-name. then just use the ip-address as the host name instead of unassigned for both. Not sure why that was happening