It occurs to me to wonder why smtpd is receiving 8bit data at all.  It isn't advertising the 8BITMIME capability, so the client should be sending only 7bit data.  See also issue 19662.  I'm not sure that making utf-8 decoding work better is a good idea, but I suppose that since it is a data-dependent exception in a situation that would work with different data, we should go ahead and fix it.