Problem
After restarting a node, journal logs from the previous run disappear.
Solution
It is essential for troubleshooting that journald preserves its logs between restarts of the systemd units or the host.
Please, check that the Storage parameter in the system journal configuration is set either to persistent or auto. If it's set to auto, please, make sure that the /var/log/journal directory exists on the host. If it doesn't yet exist, then journald writes logs to the /run/log/journal directory and the content in this directory may not survive a restart of the node due to the transient nature of the /run directory.
You can find more information about journald configuration in this article.