Pipelining isn't what I'm seeing. Two separate people (let's call them Bob and Joe) are making the requests from two different sources at the same time and each match two different conditions in the iRule, only one of which needs the timeout changed. So, connection 2 is stomping on connection 1. The $update_serverside_idle_timeout variable is the "same" between the two. I'd expect there to be one $update_serverside_idle_timeout variable for Bob's session, and another $update_serverside_idle_timeout variable for Joe's session.
No proxy server, OneConnect is not enabled on the VIP but OneConnect Transformations is enabled on the standard http profile.
Thank you,
Doug