dovecot - Feb 2015 - [Bug ?] IMAP keywords are not preserved during mailbox conversion

IMAP keywords (Thunderbird Tags) are not preserved during Maildir to mdbox
conversion.

For mailbox conversion I am using the command:
# doveadm -o mail_location=maildir:~/Maildir sync -u user at example.com
mdbox:~/mdbox

After conversion all Thunderbird Tags are lost.

Is it a bug or expected behavior?

--
Alexander
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# doveconf -n
# 2.2.15: /usr/local/etc/dovecot/dovecot.conf
# Pigeonhole version 0.4.6 (3e924b1b6c5c+)
# OS: FreeBSD 10.1-RELEASE i386
auth_default_realm = example.com
auth_mechanisms = digest-md5 cram-md5 apop plain
doveadm_password = XXXXXXXXXXXX
first_valid_gid = 1000
first_valid_uid = 1000
lda_mailbox_autosubscribe = yes
listen = *
mail_attachment_dir = /vmail/attachments
mail_gid = vmail
mail_home = /vmail/%d/%n
mail_location = mdbox:~/mdbox
mail_plugins = quota zlib
mail_uid = vmail
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope encoded-character
vacation subaddress comparator-i;ascii-numeric relational regex imap4flags copy
include variables body enotify environment mailbox date ihave duplicate
namespace inbox {
  inbox = yes
  location   mailbox Drafts {
    special_use = \Drafts
  }
  mailbox Junk {
    auto = subscribe
    special_use = \Junk
  }
  mailbox Sent {
    special_use = \Sent
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix   separator = /
}
passdb {
  args = scheme=plain username_format=%n
/usr/local/etc/dovecot/dovecot.auth/%d.passwd
  driver = passwd-file
}
plugin {
  antispam_backend = mailtrain
  antispam_mail_notspam = learn_ham
  antispam_mail_sendmail = /usr/local/bin/rspamc
  antispam_mail_sendmail_args = -h;localhost:11334
  antispam_mail_spam = learn_spam
  antispam_spam = Junk
  antispam_trash = Trash
  quota = dict:User quota::file:%h/dovecot-quota
  quota_rule = *:storage=2G
  quota_rule2 = Trash:storage=+10%%
  quota_rule3 = Spam:storage=+20%%
  quota_status_nouser = DUNNO
  quota_status_success = DUNNO
  sieve_after = /usr/local/etc/dovecot/sieve/sieve.after
  sieve_before = /usr/local/etc/dovecot/sieve/sieve.before
  sieve_vacation_min_period = 0
  zlib_save = gz
  zlib_save_level = 3
}
postmaster_address = postmaster at example.com
protocols = imap pop3 lmtp sieve
quota_full_tempfail = yes
service auth {
  unix_listener /var/spool/postfix/private/auth {
    group = postfix
    mode = 0660
    user = postfix
  }
  unix_listener auth-userdb {
    user = vmail
  }
}
service config {
  unix_listener config {
    mode = 0600
    user = vmail
  }
}
service lmtp {
  unix_listener /var/spool/postfix/private/dovecot-lmtp {
    group = postfix
    mode = 0660
    user = postfix
  }
  user = vmail
}
service quota-status {
  client_limit = 1
  executable = quota-status -p postfix
  unix_listener /var/spool/postfix/private/quota-status {
    user = postfix
  }
}
ssl_cert = </etc/ssl/certs/mx.example.com.crt
ssl_key = </etc/ssl/private/mx.example.com.key
userdb {
  args = username_format=%n /usr/local/etc/dovecot/dovecot.auth/%d.passwd
  driver = passwd-file
}
verbose_proctitle = yes
protocol imap {
  mail_plugins = quota zlib antispam imap_quota imap_zlib
}
protocol lmtp {
  mail_plugins = quota zlib sieve
}
#