bogofilter not launched on exit summary buffer

[Julien Cubizolles <j.cubizolles@free.fr>]

I noticed today that for some reason, spam processing doesn't work
anymore for me. If I mark an article as spam (M-d, $ mark) in a buffer,
it isn't processed by bogofilter and isn't moved to the spam group on
summary buffer exit. I don't remember changing anything relevant to spam
processing lately and bogofilter used to work just fine as of a few days
ago.

Here are the group parameters (the group is an nnimap one) :

((uidvalidity . "1")
 (spam-process
  ((spam spam-use-bogofilter)))
 (spam-autodetect . t)
 (spam-autodetect-methods spam-use-bogofilter)
 (spam-process-destination . "nnimap+free:Junk")
 (spam-marks
  (gnus-spam-mark)))

And this is the only message when exiting the Summary buffer:

Exiting summary buffer and applying spam rules

As you can see there is no mention of bogofilter being
called. Bogofilter is by the way used successfully for the
spam-splitting of this same nnimap group.

Julien, desperate for ideas.