The answer was indeed nothing to do with the format of the dates or an issue with the timestamp, but rather the setting in the props.conf of MAX_DAYS_AGO=2000. The problem dates where all related to records from August 2008, which coincided with the default parameter setting of 2000 days.
After initially changing it, the issue didn't immediately fix, however a restart of the services and everything now appears to be working.
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