Oh right! Forgot about that small (big) detail. The default Ubuntu MATE installation does not create /var/log/journal. So, the Storage=auto option in /etc/systemd/journald.conf falls back to volatile logs. All you have to do to enabled persistent logs is to create the directory where these logs are stored:
$ sudo mkdir /var/log/journal
--
Note 1: You can periodically run $ journalctl --vacuum-time=1month to clean up journal files older than one month, for example. Or --vacuum-size=100M to clear the directory to a maximum of 100 MB. You can also find size limitation options on the journald configuration file. See man journald.conf for details.
Note 2: As I was rummaging around my Ubuntu MATE trying to understand why you didn't have persistent logs, I also bumped into something I had forgotten when I suggested the command on my earlier post. The -n and -r options are currently not working well together due to a bug. So I want you to use the following command instead:
$ journalctl -rb -1
I'm also editing that post to remove the -n option.