As additioal info. it seems that the additional period only appears if dhclient is called for DHCP-IPv6 (stateful or stateless), such as in the following /etc/network/interfaces stanza :
iface enp4s0 inet dhcp iface enp4s0 inet6 auto privext 2 dhcp 1 If the inet6 section is commented out, no extra period appears. Martin-Éric
