DRAKMA:HTTP-REQUEST drops some input characters

test.lisp

(ql:quickload :cl-ppcre)
(ql:quickload :bordeaux-threads)
(ql:quickload :usocket)
(ql:quickload :drakma)

(asdf:component-version (asdf:find-system :drakma))
;=> "1.2.7"

(flet ((echo-content ()
         (usocket:with-socket-listener (socket "127.0.0.1" 8080 :reuse-address t
                                               :element-type '(unsigned-byte 8))
           (let ((socket (usocket:socket-accept socket)))
             (with-open-stream (stream (usocket:socket-stream socket))
               (let ((stream (flexi-streams:make-flexi-stream stream)))
                 (read-line stream)     ; Skip Request-Line
                 (read-line stream)     ; Skip Host
                 (read-line stream)     ; Skip User-Agent
                 (read-line stream)     ; Skip Accept
                 (read-line stream)     ; Skip Connection
                 (read-line stream)     ; Skip Content-Type
                 (multiple-value-bind (_ regs)
                     (ppcre:scan-to-strings "^Content-Length: (\\d+)" (read-line stream))
                   (declare (ignore _))
                   (let ((length (read-from-string (aref regs 0)))
                         (crlf (format nil "~c~c" #\return #\linefeed)))
                     (read-line stream) ; Skip CRLF
                     (format stream "HTTP/1.1 200 OK~a" crlf)
                     (format stream "Content-Type: text/plain; charset=UTF-8~a" crlf)
                     (format stream "Content-Length: ~a~a" length crlf)
                     (format stream "~a" crlf)
                     (dotimes (_ length)
                       (write-byte (read-byte stream) stream))
                     (force-output stream)))))))))
  (bordeaux-threads:make-thread #'echo-content))
               
(let ((drakma:*drakma-default-external-format* :utf-8))
  (drakma:http-request "http://127.0.0.1:8080"
                       :method :put
                       :content-type "text/plain; charset=UTF-8"
                       :content-length t
                       :content "日本語"))
;=> "日",
;   200,
;   ((:CONTENT-TYPE . "text/plain; charset=UTF-8") (:CONTENT-LENGTH . "3")),
;   #<URI http://127.0.0.1:8080/>,
;   #<FLEXI-STREAMS:FLEXI-IO-STREAM #x192793C6>,
;   T,
;   "OK"