mail-mode: still guessing big5 in utf-8 environment?

[Dan Jacobson]

Why does this still send big5 encoded messages, even though the
modeline says "u" meaning utf-8?
$ emacs -Q
C-x m (runs the command compose-mail)
enter some Chinese subject and body, and hit C-c C-c to send.

env was
LANG=zh_TW.utf8
LC_COLLATE=C
LC_CTYPE=zh_TW.utf8
LC_MESSAGES=C
address@hidden

I notice if I first write the buffer, and then send it, it gets sent
as utf-8, as I wished. Also message-mode doesn't have the problem,
only mail-mode.

Is it detecting that xcin is somehow big5 related?