How can I grab an HTTP header on CACHE_REQUEST?

Wow. That's totally weird. From the timestamps it's clear that the response that was logged (and hence excluded from the cache) was not the same one as the one that was cached. Since the hostname is the same, do you have any other rule that might be doing a CACHE::enable on the same virtual? Presumably you'd have checked for that, though. The only other thing I can think of is if somehow there was no content-length header (e.g. via "Connection: close" or chunked content) for a response then the iRule check wouldn't fire.

How about a debugging iRule like:

rule debugcache {
  when HTTP_RESPONSE {
    set size [HTTP::header "Content-Length"]
    if { [HTTP::uri] contains "/blank.htm" } {
      log "Got blank.html of apparent length $size"
    }
  }
}