It sounds like the iRule might be working correctly, but an existing serverside connection is being re-used. Do you have a OneConnect profile associated with the VS? Are the serverside connections being SNAT'd? If you don't have OneConnect enabled, can you create a custom OneConnect profile and add it to the VS? If you're using SNAT, then leave the source mask set to 0.0.0.0 to reuse the serverside connections for all clients. If you don't use SNAT, then set the source mask to 255.255.255.255.
For details on OneConnect, you can check the wiki page:
OneConnect
http://devcentral.f5.com/wiki/default.aspx/AdvDesignConfig/oneconnect.html
Aaron