dsync annoyances

Hi!

Every time I run dsync for a user against an old Cyrus server, the users mailbox in Dovecot increases by several hundred MB, these are not new emails. So the first dsync run gets the whole mailbox and subsequent runs get a bunch of duplicates or what? doveadm purge clears them all out but why is dsync behaving like this? I didn't happen in previous versions (2.2.13)