HTTP::collect does not function as expected

iRule written below does not collect the specified amount (1024 Bytes) of payload data. Am I missing something here? When I debug the payload contents, the chunked payload size amounts to only about 869 Bytes, while the specified content_length is 1024 Bytes. The size of the actual length is around 2.55 KB, lager than the specified length.

The HTTP::method is POST

BIG-IP LTM 1600

TMOS 12.1.0

when HTTP_REQUEST {
        if { [HTTP::host] contains "test-main" } {
           pool pool_test-main
           return
        } elseif { [HTTP::host] contains "test01" } {
                 pool pool_test01
                 return
        } else {
                pool pool_test02

                if {[HTTP::uri] contains "web_dummy"} {
                    if {[HTTP::header "Content-Length"] ne "" && [HTTP::header "Content-Length"] <= 1024}{
                        set content_length [HTTP::header "Content-Length"]
                    } else {
                        set content_length 1024
                    }

                     collect
                    HTTP::collect $content_length                
                } 
                return
        }
}

when HTTP_REQUEST_DATA {
       log local0. "collected data.....POST Data: [HTTP::payload]"
       foreach x [split [HTTP::payload] "&"] {
       if { $x starts_with "user_ref=" } {
            set user_ref [lindex [split $x "="] 1]
            if { [matchclass $user_ref equals CX_SERVICE_ID] }{
               HTTP::uri [string map {"/web_dummy" "/notify_dummy"} [HTTP::uri]]
               pool pool_test00
               return
               }
           }
       }
    return
}