Status messages of ddclient go to stdout, which is block buffered by
default, so most of its output won't appear in the journal until it is
exiting:
-- Reboot --
Apr 19 11:31:04 xen systemd[1]: Starting Dynamic DNS Client...
Apr 19 11:31:04 xen systemd[1]: Started Dynamic DNS Client.
Apr 19 11:31:53 xen systemd[1]: Stopping Dynamic DNS Client...
Apr 19 11:31:53 xen ddclient[530]: === opt ====
Apr 19 11:31:53 xen ddclient[530]: opt{cache} : <undefined>
Apr 19 11:31:53 xen ddclient[530]: opt{cmd} : <undefined>
...
Fix this by adding a patch to set line buffering on stdout.
7532c5379d