I thought you were getting fields from a user. See this doc https://www.advancedcustomfields.com/resources/how-to-get-values-from-a-user/
Transients would be a better way to go. I would set a different transient for each user with a small expiration time. If the transient has expired then the user in no longer logged in. Updated it each time the user interacts with a page or build a JavaScript/AJAX script that updates the transient as long at user has the site open.
