Why is source-fn failing to find source code in this specific circumstance?

This situation is quite tricky to reproduce. First I create a clj file containing:

(ns myns)

(defn myfn [x] x)

Then I create a second clj file containing:

(ns myns2
 (:require [myns :as m]
           [clojure.repl :as repl]))

(comment

 (second (iterate repl/source-fn 'm/myfn))

 (take 2 (iterate repl/source-fn 'm/myfn))

)

Then I start a REPL and load the second file in that. Finally I evaluate both comments by sending them to the REPL. The first expression will yield "(defn myfn [x] x)" as expected. However the second expression yields '(m/myfn nil). What is going on here?

Note that fully qualifying 'm/myfn as 'myns/myfn restores matching behavior. Also I understand that iterating source-fn is kinda wacky, but it's the simplest way I know to reproduce the behavior.