bug#20610: 25.0.50; Gnus fancy mail splitting not working anymore

[Sebastien Vauban <sva-news-D0wtAvR13HarG/iDocfnWg@public.gmane.org>]

While "fancy mail splitting" (section 6.4.6 of the manual) with the
`gnus-private' attribute of BBDB [1] works perfectly -- for years! --
under (Windows) Emacs 24.5 (and many previous versions), it stops
working when using Emacs 25.0.50 [2]... while (let's be explicit!) the
`.gnus' file stays strictly the same.

Here a minimal working configuration file for Emacs 24.5:

--8<---------------cut here---------------start------------->8---
(require 'gnus)

(require 'auth-source)
(setq auth-source-debug t)

;; Configure incoming mail.
(setq gnus-select-method
      '(nnimap "mail"
               (nnimap-address "mail")
               (nnimap-server-port 993)
               (nnimap-stream ssl)))

;; Name(s) of IMAP mailboxes to split mail from.
(setq nnimap-inbox "INBOX")

;; The first match in `nnmail-split-rule' found will be used.
(setq nnmail-crosspost nil)

;; BBDB (Big Brother DataBase) is loaded from my `.emacs' file.
(require 'bbdb-gnus)

;; Split function to use (sorting mails into groups using BBDB).
(setq nnimap-split-methods 'nnimap-split-fancy)

;; Specify how to split mail.
(setq nnimap-split-fancy
      '(|                           ; split to the *first* match
          ;; Invoke BBDB.
          (: (lambda ()
               (car (bbdb/gnus-split-method))))

          ;; Catch spam.
          ("X-Spam-Status" "[Yy]es"
           "INBOX.Spam")

          ;; Undecided.
          "INBOX"))

;; For records which don't have `gnus-private' set, the rules in
;; `split-fancy' are invoked.
(setq bbdb/gnus-split-default-group nil)

;;; .gnus ends here
--8<---------------cut here---------------end--------------->8---

I've tried numerous changes (for hours), such as:

- renaming `nnimap-split-methods' to `nnmail-split-methods' and
  `nnimap-split-fancy' to `nnmail-split-fancy', or

- moving `nnimap-inbox' and `nnimap-split-methods' (without the `setq'
  and without the quotes) inside the definition of `gnus-select-method',

with no success. In those 2 latter cases, it even stops working within
Emacs 24.5, so this seems not to be the right path...

Best regards.

[1] ~/.emacs.d/elpa/bbdb-20140830.2031/bbdb-gnus.elc

[2] GNU Emacs 25.0.50.1 (i686-pc-mingw32) of 2015-05-08 on LEG570