Up until version 4.12, when #syslog_ng ran into an error jumping to a saved #systemd #journal position, it read the journal from the beginning. With the latest syslog-ng version, you can configure what happens in case of such an error.
https://www.syslog-ng.com/community/b/blog/posts/syslog-ng-journald-source-how-to-avoid-log-bombs-on-errors