An issue with core.logic prep?

gistfile1.clj

(run* [q]
    (fresh [pattern production guards rule rule-name]
      (membero
       {:pattern pattern :production production}
       [{:pattern (partial-map  {:a :clojure.core.logic/not-found :x 1})
         :production {:a 1}}
        {:pattern (partial-map  {:a '?a :c :clojure.core.logic/not-found})
         :production {:c '(* ?a 2)}}])
      (== {:x 1}  pattern)
      (== q production)))
  => ({:a 1})

(run* [q]
    (fresh [pattern production guards rule rule-name]
      (membero
       {:pattern pattern :production production}
       [{:pattern (partial-map (prep {:a :clojure.core.logic/not-found :x 1}))
         :production {:a 1}}
        {:pattern (partial-map (prep {:a '?a :c :clojure.core.logic/not-found}))
         :production {:c '(* ?a 2)}}])
      (== {:x 1}  pattern)
      (== q production)))
  => ({:a 1} {:c (* ?a 2)})

The above are identical apart from the use of prep.

(let [rules
      [{:pattern (partial-map (prep {:a :clojure.core.logic/not-found :x 1}))
        :production {:a 1}}
       {:pattern (partial-map (prep {:a '?a :c :clojure.core.logic/not-found}))
        :production {:c '(* ?a 2)}}]]
  (run* [q]
    (fresh [pattern production guards rule rule-name]
      (membero {:pattern pattern :production production} rules)
      (== {:x 1}  pattern)
      (== q production))))
  => ({:a 1} {:c (* ?a 2)})