iRule: Referer header not removed in HTTP request

In order to test a possible solution for a problem, I want to remove the Referer header for certain incoming requests. This is the irule I created and added to the impacted virtual server:

when HTTP_REQUEST { 
     if { ( [HTTP::uri] contains "abc") && ( [HTTP::header "Referer"] contains "def")} {
              log local0. [HTTP::header "Referer"]
              HTTP::header remove "Referer"
              log local0. [HTTP::header "Referer"]
     }
}

However, when I take a tcpdump on the SNAT address of the virtual server, I can still see the Referer header with content being sent to our webservers. The logs above do show the Referer content being erased

Version 11.5.4