The correct way to reply to a message that has an RFC 2047-encoded subject
is to use the same RFC 2047-encoded subject that appeared in the message
being replied-to. That's very easy to do - just prepend "Re: " to that
subject line, and rewrap the header (leaving encoded-words intact) if
necessary to accomodate RFC 2047 line-length restrictions.
Suppose that a mail client receives a message whose subject is encoded
according to RFC 2047. Suppose further that the message is decoded and
stored in a database somewhere - in RAM, on disk, on a server
somewhere.
RFC 2047 encoded words were not intended to be used in this way. Decoding
is only supposed to happen prior to display or printing, not prior to
storage.
Keith