UTF-8 character boundaries mis-recognized with flexi-streams, on SBCL

character-boundary.lisp

(ql:quickload '(:flexi-streams :circular-streams))

(setf s "こんにちは")
(setf o (flex:string-to-octets s :external-format :utf-8))

(with-input-from-string (in s)
  (format t "read from stream: ~s~%" (read-char in)))
;; => read from stream: #\HIRAGANA_LETTER_KO

(flex:with-input-from-sequence (in o)
  (format t "read from circular-stream: ~s~%"
          (trivial-gray-streams:stream-read-char
           (flex:make-flexi-stream in))))
;; => read from circular-stream: #\LATIN_SMALL_LETTER_A_WITH_TILDE

(print (code-char (aref o 0)))
;; => #\LATIN_SMALL_LETTER_A_WITH_TILDE