i tried running

 emacs -Q -f notmuch

and replying to a message.

Since i didn't have my emacs config, it wanted to create the default
sent folder in ~/mail/sent

if i told it "no, don't create it" when prompted (during the setup of
the reply message) then the body of the replied-to message doesn't show up.

If ~/mail/sent is actually a file (not a folder) so that the fcc folder
cannot be created, then i get the same problem (the replied-to message
doesn't show up quoted in the body of the new *mail* buffer).

sorry i haven't been able to track it down more than that.

to replicate:

 mv ~/mail/sent ~/mail/sent.bak
 touch ~/mail/sent
 emacs -Q -f notmuch

   <find a message, hit 'r'>

hth,

	--dkg