"Excluded: Authcache varnish key callback" when loading a block

This is using Varnish 4 and ESI.

I'm not sure if this is an error, or there needs to be some improvement in the VCL file or how blocks are processed. In testing the Mini Panels patch I wrote (https://www.drupal.org/project/authcache/issues/2231701), I also ran a simple experiment where a Mini Panel is added as a block using Authcache Block.

This seemed to work without problem, since it's essentially just loading a block to the page. However, I did see one peculiar result in watchdog. Every time a block would be initially loaded, it threw the following error:

User: test_user
Location: http://testsite.io/authcache-varnish-get-key
Message: Excluded: Authcache varnish key callback

I added a watchdog message at the top of authcache_p13n_frontcontroller_handle_request, and it returned what I expected for $req:

$req = Array
(
    [a] => 
    [r] => frag/block/panels_mini-header_above
)

Also the important $_SERVER values at the start of authcache_p13n_frontcontroller_handle_request seemed to be correct:

    [SCRIPT_FILENAME] => /app/authcache.php
    [QUERY_STRING] => a=&r=frag/block/panels_mini-header_above&o%5Bq%5D=front
    [REQUEST_METHOD] => GET
    [SCRIPT_NAME] => /authcache.php
    [REQUEST_URI] => /authcache.php?a=&r=frag/block/panels_mini-header_above&o%5Bq%5D=front
    [DOCUMENT_URI] => /authcache.php
    [HTTP_COOKIE] => SESSb25cb6e5f7953f35d59a6b9a07ea7b5c=t4zEI8kBrBe4Onatu-e45DdR6MnBgaw3hnLbZ9LzSBk; aucp13n=9bcpn6
    [HTTP_X_AUTHCACHE_DO_ESI] => 1
    [HTTP_X_AUTHCACHE] => 1
    [HTTP_X_AUTHCACHE_KEY] => 3996d18
    [HTTP_X_AUTHCACHE_GET_KEY] => done
    [HTTP_X_VARNISH] => 819745
    [HTTP_X_AUTHCACHE_VARNISH_PASSPHRASE] => correct horse battery staple
    [PHP_SELF] => /authcache.php

What's also strange is the second block loaded the warning in watchdog with a user of "Anonymous":

User: Anonymous (not verified)
Location: http://testsite.io/authcache-varnish-get-key
Message: Excluded: Authcache varnish key callback

But the $req and $_SERVER watchdog statements after this "Exclude" returned the correct test_user user.

Any thoughts on what could be causing this, or where to investigate? The page is still being cached and receive a HIT from Varnish, but the message doesn't make sense.