Hi, @gubbins
Before diving deeper into this issue, can you please go through the Mattermost Is Not Working / The Server Keeps Dying and provide the output of the following commands when the issue occurs?
sudo systemctl status mattermost.service
sudo journalctl -u mattermost.service
Additionally, is the behavior constantly reproducible whenever you shut down the server using systemd? Any recent changes performed on the system prior to the observation?