As a follow-up to this post from two years ago (!!) How I use Apache behind pfSense, I think I may have solved a similar problem with HTTP authentication, trac, and the pfSense sticky states balancer.
Hmmm, its very strange because it just switched again. I wish I knew why it was not remaining a sticky connection. The blurb doc says as long as there is a connection, it stays sticky. Hence the keepalive, but its apparently not working. :-(
Aha - I think I may have figured it out. I was connecting to other back-ends through the load balancer which had different keepalive settings. I wish the keepalive settings worked.
There are many facets to this situation: http keepalive, tcp keepalive, pf states, proxies, and more I'm not thinking of at the moment. Its very interesting though.