[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@example.com mdbox:~/mdbox

After conversion all Thunderbird Tags are lost.

Is it a bug or expected behavior?

-- Alexander