SSL persistence not working correctly in v11.4.1

SSL persistence uses the SSL sessionid established during the handshake to maintain stickiness. This has nothing to do, more or less, with the certificate and key used to establish the SSL session, nor the application layer processing above it. I'm guessing here, but the issue I think you're seeing is the result of clients and/or the server (F5) renegotiating the SSL at semi-random points during the session. Browsers do this quite frequently, so SSL persistence is rarely ever a viable option for browser-based access. There are still some clients, particularly the Citrix agent, that don't renegotiate SSL and where SSL persistence can be used, but it may just be that Lync is renegotiating and killing SSL persistence.