If the CN is already an FQDN on the given domain, this prevents the domain from being present in the record twice (e.g. host.example.com as a CN leads to records for host.example.com.example.com)
Related issues
Feature #6847: Register CN of OpenVPN clients in DNS Resolver
Added by Jim Pingle over 7 years ago
Trim domain for learned DNS entries. Ticket #6847
If the CN is already an FQDN on the given domain, this prevents the
domain from being present in the record twice (e.g. host.example.com
as a CN leads to records for host.example.com.example.com)