I am trying to convert my app from ACF - 2018 to Lucee 5.3.8, but I can't see to get onSessionStart to trigger. Any suggestions?

How do you know your event isn't firing? What are you doing to try to confirm that it's running? Are you writing to a log file? Did you check the catalina.out log files to make sure there's not an exception being triggered in the event handler? Maybe it's running, but just throwing errors before your test code runs.