The 5 minute retention of Bulletins is a hard coded value that cannot be edited by the end user. It is normal to see the occasional bulletin from some NiFi processors. For example a failed putSFTP because of filename conflict or network issues, but on retry it is successful. A continuous problem would result in non stop bulletins being produced which would be easily noticed.
Take a look at my response further up on using the "SiteToSiteBulletinReportingTask" if you are looking to retain bulletins info longer, manipulate, route, store that somewhere, etc..
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