expire httpOnly cookie is request

I just tried it and it works for me with HTTPOnly - the F5 wouldn't care about that anyway - it's a cookie attribute and we've asked to remove the cookie, so it would.

Try this in HTTP_REQUEST - can you see the cookie in the request in the log afterwards?

if {[HTTP::cookie exists "mycookie"]} {
    HTTP::cookie remove "mycookie"
    log local0. [HTTP::request]
}