Dovecot version: 2.2.9

I'm using doveadm backup to migrate about hundred user mailboxes from cyrus to dovecot. I defined a master user for the old cyrus mailserver in imapc backend to get access to all the mailboxes. Now I realized that after migration every user's "Inbox" or "Trash" on the new dovecot server has the same mailbox-guid, e.g:

User a: inbox dd9456... trash 5549cc...

User b: inbox dd9456... trash 5549cc...

Is this a wanted behaviour? If i do not migrate with doveadm backup and just manually create mailboxes, the mailbox-guids are globally unique, e.g:

User a: inbox dd9456... trash 5549cc...

User b: inbox c543fe... trash t49ccc...

Thanks! Christian