[Dovecot] Undeserved permissions error with g+s folder permissions

[tlhackque]

I have a (mbox/IMAP) directory under my mail directory with these 
permissions:

drwxrws--- 4 tlhackque mail 4096 Dec  1 10:34 Vendors/

It contains a subdirectory:

-rw-rw---- 1 thlackque mail 84805345 Dec  1 10:34 Vendors/AVendor

If I try to rename the AVendor folder (client is Thunderbird 17.0), I 
get this error:

     Unable to rename across conflicting directory permisssions

If I chmod g-s Vendors, the RENAME command succeeds.

I shouldn't have to do this...

Version and configuration information:

dovecot --version
2.1.10

# OS: Linux 2.6.22.14-72.fc6 i686 Fedora Core release 6 (Zod)
first_valid_gid = 4000
first_valid_uid = 4000
hostname = example.net
login_greeting = Dovecot ready.  Unauthorized access is prohibited.
mail_access_groups = mail
mail_location = mbox:~/mail:INBOX=/var/mail/%n
mail_privileged_group = mail
namespace inbox {
   inbox = yes
   location =
   mailbox Drafts {
     special_use = \Drafts
   }
   mailbox Junk {
     special_use = \Junk
   }
   mailbox Sent {
     special_use = \Sent
   }
   mailbox "Sent Messages" {
     special_use = \Sent
   }
   mailbox Trash {
     special_use = \Trash
   }
   prefix =
   separator = /
}
passdb {
   driver = pam
}
plugin {
   autocreate = Trash
   autocreate2 = Train As Spam
   autocreate3 = Train As Ham
   autocreate4 = Train As Forgotten
   autosubscribe = Trash
   autosubscribe2 = Train As Spam
   autosubscribe3 = Train As Ham
   autosubscribe4 = Train As Forgotten
}
service imap-login {
   inet_listener imap {
     address = imap.v4.example.net imap.v6.example.net
     port = 143
   }
   inet_listener imaps {
     address = imap.v4.example.net imap.v6.example.net
     port = 993
     ssl = yes
   }
}
service pop3-login {
   inet_listener pop3 {
     address = pop.v4.example.net pop.v6.example.net
     port = 110
   }
   inet_listener pop3s {
     address = pop.v4.example.net pop.v6.example.net
     port = 995
     ssl = yes
   }
}
ssl_ca = </etc/pki/tls/certs/example_net_ca-cert-crl.pem
ssl_cert = </etc/pki/tls/certs/servers-example-net.pem
ssl_key = </etc/pki/tls/private/servers-example-net.pmk
userdb {
   driver = passwd
}
protocol imap {
   imap_client_workarounds = tb-extra-mailbox-sep tb-lsub-flags
   mail_plugins = " autocreate"
}
protocol pop3 {
   mail_plugins = " autocreate"
   pop3_client_workarounds = outlook-no-nuls oe-ns-eoh
   pop3_uidl_format = %08Xu%08Xv
}

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if any, on the matters discussed.