@MeAndMyBIGIP
I could set the hostname using CLI to my_new_hostname.domain.com and my_new_hostname.subname.domain.com but not from GUI:
[root@my_new:Active] config b system hostname my_new_hostname.domain.com
[root@my_new_hostname:Active] config b system hostname
SYSTEM - Local Host Name: my_new_hostname.domain.com
[root@my_new_hostname:Active] config b system hostname my_new_hostname.subname.domain.com
[root@my_new_hostname:Active] config b system hostname
SYSTEM - Local Host Name: my_new_hostname.subname.domain.com
I'm not sure whether or not GUI's error is a bug. I personally will not use underscore in the hostname as it's not compliant with the RFC.