Comment 8 for bug 1278193

I've went through this problem just now. The changes described in #5 solved the problem for me as well.

The fix released in LP: #1258202 (logrotate version 3.8.6-1ubuntu2) adds those lines to the default logrotate.conf file, but I got some custom configs in mine and so I had to manually change it.

Of course it only works when it reads logrotate.conf, which I guess is not the case when you run this way:
 logrotate -df /etc/logrotate.d/rsyslog

Running either /etc/cron.daily/logrotate or "/usr/sbin/logrotate /etc/logrotate.conf" works fine. Maybe the config "su root syslog" should be the compile-time-default in Ubuntu so to avoid this situation. Don't know if it is possible.

Also, from what I understand reading man logrotate.conf, the "su root syslog" line only changes the user/group that the process of rotation will run as, which does not (necessarily) affect the owner and group of the log files: that is defined by the "create" directive (see the logrotate.conf man for that directive). That's why you don't see any files with group syslog, Uwe.