How is it possible that loop-recur throws a StackOverflowError?

I´ve been trying to implement a tail-recursive version of chainl1, but even with the loop-recur it throws a StackOverflowError. How is that possible and what can i do to change that?

(defn atest [state]
   (when-not (and (= "" state) (not (= (first state) \a))) 
       (list (first state) (. state (substring 1)))))
 (defn op [state]
   (when-not (and (= "" state) (not (= (first state) \a)))
     (list #(list :| %1 %2) (. state (substring 1)))))
 (defn chainl1-helper [x p op]
   (fn [state]
     (loop [x x
            state state]
       (if-let [xs (op state)]
         (when-let [xs2 (p (second xs))]
           (recur ((first xs) x (first xs2)) (second xs2)))
         (list x state)))))
 
 (defn chainl1 [p op]
   (fn [state]
     (when-let [[v s] (p state)]
       ((chainl1-helper v p op) s))))
 (def test-parse (chainl1 atest op))
 (defn stress-test [n] (test-parse (apply str (take n (interleave (repeat "a") (repeat "+"))))))
 (stress-test 99999)
 

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