All,
I finally recognized that I did not state the problem properly. The title
should be: "Articles appear out of order", and the description should
read: in the summary buffer, `gnus-group-select-group' shows the
articles in the wrong order, both through IMAP and Maildir, despite
(setopt gnus-thread-sort-functions
'(gnus-thread-sort-by-most-recent-date)),
being present. The Maildir is sync'd up with the IMAP account through
`mbsync'.
On the Maildir side, the solution was a matter of deleting the
.nnmaildir sub directory and letting GNUS re-populate it.
Since I have all the articles archived locally and deleted them from the
server, I don't know what would happen if I were to sync the Maildir
back to the IMAP account. My guess is that the order would fix itself
there too.