HSTS - Header not inserted with iRule

This can also be caused by Web Acceleration or manual use of iRule CACHE commands.

e.g. HTTP_RESPONSE will not fire on anything pulled from cache.

You can use CACHE_RESPONSE instead. I now use two rules and attach as appropriate.

when HTTP_RESPONSE {
    HTTP::header replace Strict-Transport-Security "max-age=31536000; includeSubDomains"
}

And,

when CACHE_RESPONSE {
    CACHE::header replace Strict-Transport-Security "max-age=31536000; includeSubDomains"
}