Information
Data from journald may be stored in volatile memory or persisted locally on the server. Logs in memory will be lost upon a system reboot. By persisting logs to local disk on the server they are protected from loss.
Note: The main configuration file /etc/systemd/journald.conf is read before any of the custom *.conf files. If there are custom configs present, they override the main configuration parameters
Writing log data to disk will provide the ability to forensically reconstruct events which may have impacted the operations or security of a system even after a system crash or reboot.
Solution
Edit the /etc/systemd/journald.conf file and add the following line:
Storage=persistent