Subdomain doesn't resolve after adding to zone due to missing "."

When adding a subdomain, either in WHM or in some external DNS panels, the domain will continue to not resolve. In the "Name," "Domain," "Key," or "Domain" field, the full domain is entered in the following format.

sub.domain.com

Description 

DNS zones are relative, and the full domain is typically not required to add a subdomain. Without the "root" indicator, it is assumed that the entirety of what is entered into the field is the subdomain, meaning the following likely resolves:

sub.domain.com.domain.com

Workaround

If you would like to enter the fully-qualified domain name, rather than just the subdomain, the "root" indicator must be used.

sub.domain.com.

Otherwise, only the subdomain must be entered

sub