Cookie Encyrption Question - I'm confused

if you read the Tenable article:

https://www.tenable.com/plugins/nessus/20089

then it does seem to indicate this is about the persistence cookie, so it is weird you say persistence isn't used.

can the security team perhaps show some "proof" about which cookie and for which virtual server this is?

if it is about the F5 cookie, then you don't have to change anything on the http profile, as that is for the backend server cookies.