Problem with session persistence using CARP when load balancing a McAfee Web Gateway cluster using progress page for downloads

Yes, I am using the HTTP::host as hash. Here's my irule:

when HTTP_REQUEST {
  set debug 0
  set host [HTTP::host]
  set client [IP::client_addr]

  if { $host ne "" } {
    if { $debug == 1 } { log local0. "Persisting on host $host" }
    persist carp $host
  } else {
    if { $debug == 2 } { log local0. "Persisting on client $client, User-Agent is: [HTTP::header value User-Agent]" }
    persist carp $client
  }
}

As long as there's a host header it will hash that, otherwise it will hash the source ip. It works fine when I test it on a separate virtual server, but once I try it out in production it doesn't work reliably. The HTTP::host for the progress page is the same as for the download page.

CARP persistence should not keep any records, right? Could it be that it somehow remembers the previous persistence even after I switch to my CARP hash profile?