Comment 1 for bug 237976

It's a bug in reading from files (refilling the buffers in the middle of a multibyte character) I believe since it does not show up if you read the file as a bytevector and decode the bytevector as in:

      (let ([r (call-with-port
                 (transcoded-port
                   (open-bytevector-input-port
                     (call-with-port
                       (open-file-input-port tmp-file (file-options)
                         (buffer-mode block)
                         #f)
                       get-bytevector-all))
                   transcoder)
                 get-string-all)])