Why is my test.check test using inputs smaller than the generator?

Question

I've implemented my own version of index-of. It takes two strings and returns the index where the second string is found in the first. In use, it seems to work fine, but it's failing the test.check property test. The weird thing is test.check says it fails at values like "0bU", but I have it set so the smallest input is 100 characters. Why is it using inputs smaller than what I've told it to use?

I wrote my own generator using gen/fmap and it looks correct when I call it using gen/sample.

The code:

(defn gen-alphanumeric   "A generator that accepts a minimum and maximum string length" [min max]   
    (gen/fmap str/join (gen/vector gen/char-alphanumeric min max)))

(defspec indexof-functionality-test   
  (prop/for-all [s1 (gen-alphanumeric 100 200)
                 begin gen/nat 80]
    (let [end (+ begin 10)
          s2 (subs s1 begin end)]
      (= (str/index-of s1 s2) (cp-str/indexof s1 s2)))))

The output from lein test

FAIL in (indexof-functionality-test) (string_test.clj:57) expected: {:result true} actual: {:shrunk {:total-nodes-visited 11, :depth 4, :pass? false, :result false, :result-data nil, :time-shrinking-ms 2, :smallest ["240"]}, :failed-after-ms 0, :num-tests 4, :seed 1676231835751, :fail ["n5n0"], :result false, :result-data nil, :failing-size 3, :pass? false, :test-var "indexof-functionality-test"}

EDIT:

My cp-str/indexof

(defn indexof
  "given two strings, returns the index where sb begins in sa. Nil if not found"
  ([sa sb]
   (indexof sa sb 0))
  ([sa sb i]
   (let [sa-len (count sa)
         sb-len (count sb)]
     (cond
       (>= 0 sa-len) nil
       (>= 0 sb-len) nil
       ; this is hit when element not found
       (> (+ i sb-len) sa-len) nil
       (= (subs sa i (+ i sb-len)) sb) i
       :else (indexof sa sb (inc i))))))

EDIT2: It fails on one computer and passes on another. Those three methods are identical on the two computers.