Ubuntu – Cannot find upstart logs in syslog despite a log-priority of debug (ubuntu 13.04 & mint 16)

I've set upstart log level to "debug"

sudo initctl log-priority debug

But I still cannot see anything in /var/log/syslog.

sudo initctl emit hello # no log
sudo initctl start my-broken-service # still no log

I've checked rsyslog config (in /etc/rsyslog.d/50-default.conf), and everything seems fine:

 *.*;auth,authpriv.none     -/var/log/syslog

Logging works

 logger hello # prints hello in /var/log/syslog

I'm running both an ubuntu 13.04 and a mint 16 and the result is the same.

I wanted to check upstart options in grub but I couldn't find a line in grub's config matching upstart.

Any idea?

Edit: This problem also occurs on Ubuntu 13.10. On Ubuntu 12.04, the log messages appear in /var/log/syslog. See this question on Ask Ubuntu.