Clojurians Log v2

I now have a concrete simple example to illustrate the thing I don't really understand regarding the completion step of transducing.

(let [xform (comp (partition-by count)
                  (filter (constantly false)))
      coll  ["aa" "bb" "ccc"]]
  (transduce xform conj [] coll))

This results in [] as one would expect from everything being filtered out. What I don't understand is how this happens. The way I'm imagining the process I would have expected filter to do nothing for the final partition. I imagine the stateful partition-by to have ["ccc"] stored internally and return it at the final completion step where no filtering happens, resulting in [["ccc"]] instead of []. Obviously that is not what happens. Here's what I can figure out by reading the source code carefully:

;; exerpt from partition-by source code

(let [v (vec (.toArray a))]
  (.clear a)
  (let [ret (rf result v)]
    (when-not (reduced? ret)
      (.add a input))
    ret))

Upon reaching "ccc", we return (rf result v) which evaluates to (conj [] ["aa" "bb"]) or simply [["aa" "bb"]] and store ["ccc"] internally. As expected. Then we enter the completion step where some magic must happen that I fail to see. All I see is that we return (rf (unreduced (rf [] ["ccc"]))) . I think that should evaluate to [["ccc"]] that then gets passed down to the completion step of filter which does nothing. What am I missing? I'm sure I'm doing this all wrong, trying to imperatively follow the path of execution, but I'm not sure how else to approach it.