variable getting reset during irule execution

Here is the entire rule with IP's changed etc. But this is the meat of it.

when HTTP_REQUEST { 
set uri [HTTP::uri]
set host_num 0
set req_cookie [HTTP::cookie "DEVPERSIST"]

if { [string tolower $uri] starts_with "/serv" } {


if { [string tolower $uri] matches_regex {serv[\d]+} } {
scan $uri /serv%d host_num
set path "/"
} else {
scan $uri /serv%d%s host_num path
}

snip the host prefix from the URI
HTTP::uri $path
                


remove the cookie if it's there
if { [HTTP::cookie exists DEVPERSIST] } {
HTTP::cookie remove DEVPERSIST
}
                
              

}
}

when LB_SELECTED {
if { $host_num != 0 } {
                log local0. "hostnum here $host_num"
redirect the request to the appropriate member
switch $host_num {
1 {
LB::reselect pool DEV member 10.10.10.2:7085
}

2 {
LB::reselect pool DEV member 10.10.10.3:8080
}

}
}
}

when HTTP_RESPONSE {
if { $host_num != 0 } {
HTTP::header insert Set-Cookie "DEVPERSIST=[HTTP::cookie DEVPERSIST]; Path=/"
            log local0. "hostnum here $host_num"
}
}

I haven't rebooted the F5 yet since it serves about 30 different high usage web applications. But I can talk to our sys admins about doing so at some point.