When openSUSE is booting up it sets the hostname from /etc/hostname without
using the domain part. (Though it's questionable that it does something else
than "hostname -F /etc/HOSTNAME", this is well known SUSE behavior.)

When starting NetworkManager it re-sets the hostname FQDN from /etc/hostname.

That's confusing and causes other programs to behave differently depending on
NetworkManager is already started or not. I don't think there is any need to
use two different hostnames during startup. And also no good reason why
traditional network setup (wicked) and NM will cause different hostnames.

As a side note NetworkManager does NOT set the hostname on "rcnetwork reload"
which should be done if changing hostname is wanted at all.