Messages marked unread after refile?

["Ian D. Leroux" <idleroux@fastmail.fm>]

I have my mail stored locally in maildirs, which get sync'd to my
mailserver using offlineimap. Whenever I refile read messages from one
folder to another (using 'o', nothing exotic) the moved messages are
marked as unread, sort of:
- The next time I update my folder list with 's', the destination
  folder's counter of unread messages increases by the number of
  messages transferred to it.
- offlineimap sees the transferred messages as unread and replicates
  them as such to the mail server.
BUT
- Wanderlust still knows the messages are read, because the moment
  I open the destination folder in wanderlust all the moved messages
  immediately get marked as read again.

It's not a huge annoyance, as I just have to remember to open the
destination folder of any message refile in Wanderlust before looking at
it with any other client, but it would be nice to just have the messages
marked correctly when refiled.

Does anyone have a clue stick to club me with?

Ian Leroux