iRule and Cookie persistence

you can decode the cookie values to determine destination. I think the likely issue is the lack of a oneconnect profile on your virtual server. Without oneconnect, the initial request in the tcp connection is load balanced to server A, and all subsequent requests within that tcp connection will also go to server A regardless of persistence because BIG-IP typically load balances connections, not requests. OneConnect overrides that behavior by effectively detaching the server side of the connection and forcing a load balancing decision on every request.

 

You can read more about OneConnect and HTTP here.