Anyone who works with systemd soon comes to realize that systemd just doesn't like syslog very much. In fact systemd is so unhappy with syslog that it invented its own logging mechanism (in the form of journald). This is not news. What people who don't have to look deeply into the situation often don't realize is that systemd's dislike is sufficiently deep that systemd just doesn't interact very well with syslog.
This is a must-read article for anyone who needs to use systemd and syslog together.