Clojure range (next) thread safety issue?

results.clj

FAIL in (range-test) (range_test.clj:18)

expected: 1000
  actual: 927
         
Ran 1 tests containing 1 assertions.
1 failures, 0 errors.
Reloading scratch.range-test ... done.

Testing scratch.range-test

FAIL in (range-test) (range_test.clj:18)

expected: 1000
  actual: 351
         
Ran 1 tests containing 1 assertions.
1 failures, 0 errors.
Reloading scratch.range-test ... done.

Testing scratch.range-test

FAIL in (range-test) (range_test.clj:18)

expected: 1000
  actual: 223

scratch-test.clj

(ns scratch.range-test
  (:require [clojure.test :refer :all]))

(deftest range-test
  (let [threads 10
        n       1000
        r       (atom (range (inc n)))
        m       (atom 0)]
    ; Iterate through the range concurrently, 
    ; updating m to the highest seen value in the range
    (->> (range threads)
         (map (fn [id]
                (future
                  (loop []
                    (when-let [r (swap! r next)]
                      (swap! m max (first r))
                      (recur))))))
         (map deref)
         dorun)
    (is (= n @m))))