lost one month's mails during archiving

[fishywang]

3.0b4. I use archive to month setting (mail.server.default.archive_granularity set to 2).

before archiving I had all the aug. and sep. mails in my inbox (previously archived up to jul.). I select all mail in inbox, and use keyboard shortcut (a) to archive them.
first the sep. mails was archived (I sort my mails in inbox in the date desc. order), I can see only aug. mails left in the inbox and the 2009-09 folder was created under archives.
then it is archiving aug. mails, very very slow (600+ sep. mails within minutes, and 700+ aug. mails in about 30 minutes). I can see the status bar showing something like "copying message xxx of xxx to 2009-08", but I never found 2009-08 in my archives, there's 2009-07 and 2009-09, but no 2009-08.
when it's finally done, my inbox is empty now. I restarted thunderbird, still can't find 2009-08. but I can see something like "Indexing 1 of 1 in 2009-08" in the status bar.

any clue? I also can't find it in the filesystem:
===
fishy@Makelele:~/Library/Thunderbird/Profiles/d67mtbv0.default/Mail/Local Folders$ find . -name "2009-07"
./Archives.sbd/2009.sbd/2009-07
fishy@Makelele:~/Library/Thunderbird/Profiles/d67mtbv0.default/Mail/Local Folders$ find . -name "2009-08"
fishy@Makelele:~/Library/Thunderbird/Profiles/d67mtbv0.default/Mail/Local Folders$ find . -name "2009-09"
./Archives.sbd/2009.sbd/2009-09
===

[fishywang]

OK I found them, under one of my imap mailbox's Archives, this is really weird, and should be a bug.