That is very strange. Are the attendees still in the database? There isn’t anything in EE that hides/removes the attendees when updating.
What kind of server are you on?
Try running the Event Espresso requirements check plugin if you’re not sure what kind of server the site is installed on.
The support post ‘Every upgrade has caused all previous attendees to be missing’ is closed to new replies.
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Support forum for Event Espresso 3 and Event Espresso 4.