Persistence Cookie Random Values

Our QA department used to test servers by using Fiddler to override the encrypted persistence cookie value....this was in 10.2.4. They would hit the VIP and go through all the servers in the pool and use Fiddler to extract their encrypted persistence cookie value. That encrypted persistence cookie value would never change. The value was the same day in and day out. We recently upgraded to 11.5.2HF1 and now it seems that there is some randomness to the encrypted persistence cookie value. The encrypted persistence cookie value it hands out for a back end server does not stay the same. If I wait thirty minutes and hit the same VIP, the encrypted persistence cookie value will not be the same. I know what server I am hitting by an identifier at the bottom of the web page. I then look in Fiddler to see what the cookie value is and it does not match the one from thirty minutes earlier. I want to make it clear that cookie persistence is working. So nothing is "broken" except for QA not being able to override cookies and force the connection to the desired server. Is this working as intended?

Thanks