mutex-lock! doesn't always lock mutex

[megane]

;; From C4 tests/mutex-test.scm
(import scheme)
(cond-expand
 (chicken-5 (import (chicken base)
		    (chicken type)
		    (chicken format)
		    srfi-18))
 (else (import chicken) (use srfi-18)))

(define-record-printer (mutex x out)
  (format out "<mutex ~a ~a~a ~a (owner ~a) waiting ~a>"
	  (mutex-name x)
	  (if (##sys#slot x 5) "LOCKED" "FREE")
	  (if (##sys#slot x 4) "/ABANDONED" "")
	  (mutex-state x)
	  (if (##sys#slot x 2) (##sys#slot x 2) "none")
	  (##sys#slot x 3)
	  ))

(let ((m1 (make-mutex)))
  ;; (print m1)
  (mutex-lock! m1)
  (print m1)
  (print* "l")
  ;; (thread-sleep! 0.001)
  (let ((t1 (thread-start! (lambda ()
			     (print* "t")
			     (mutex-lock! m1 0.001)
			     (print "\noh no, we got the mutex")
			     (print m1)
			     (exit 1)))))
    (print* "y")
    (thread-yield!)
    (print* "Y")
    (when (eq? 'ready (##sys#slot t1 3)) ;; <- #3 thread state
      (print* "X")
      (thread-sleep! 1)
      (print* "Z"))))

(print* ".")

;; OK run:
;; lytY.<mutex mutex0 LOCKED #<thread: primordial> (owner #<thread: primordial>) waiting ()>

;; Failing run:
;; lytYX
;; oh no, we got the mutex
;; <mutex mutex0 LOCKED #<thread: primordial> (owner #<thread: primordial>) waiting ()>

[megane]

Fails when compiled and in interpreter. It has taken up to 1250 tries for this to fail, which is less than a minute.