'seen' mark not preserved for nnmaildir

[Dennis Hamester]

I'm observing something rather weird with the seen mark and my
mails. Let me try to explain:

I can see that the mark is stored in my .newsrc.eld file, among others,
like expire, or reply. However, when I start gnus, all mark related
entries are loaded into gnus-newsrc-alist, except seen. If I call
gnus-read-newsrc-file after that, the seen marks appear in that alist as
well and all is fine. Well almost. Whenever I run
gnus-group-get-new-news after that, all seen marks disappear, from the
alist as well as from the .newsrc.eld file.

AFAICT this happens exclusively to the seen mark, all other marks
preserved correctly either in the .newsrc.eld file or the maildir
backend (e.g. read, expire). This also happens only to maildir groups,
not nntp groups. Their seen mark is stored and loaded. I don't know how
other mail backends would behave. I only have a single nnmaildir backend
(and nntp) and no actual mail sources.

I was also able to replicate this on a minimal gnus init file. With no
settings other that the select method. I'm not even sure this is a bug
or just intended default behavior. I tried searching all relevant gnus
and emacs mailing lists and google, but didn't come up with anything.

If you're curious why this is actually a such problem for me: I only
noticed it when I set up spam.el to check my mail for spam when entering
the summary. It works beautifully, with the exception that it checks all
my mail over and over again, unless that seen mark is preserved and
gnus-newsgroup-seen set correctly.

I really appreciate any help with this and sorry for the rather longish
post.