doveadm backup, question

[dovecot-user]

Hello everyone.
I am an inexperienced user of dovecot and I faced probably a simple 
problem, but I still do not understand how to solve it.
The issue is related to using doveadm backup - I want to backup 
mailboxes; make this copy on the same machine (hereinafter, files with 
backup copies will be taken from this machine into the storage by a 
separate mechanism, this is not related to the issue).
I am using dovecot 2.2.27 (c0f36b0) on Debian GNU/Linux 9 (stretch).
The options related to storing mail and attachments are as follows:
mail_location = mdbox:/var/mail/%d/%n
mail_attachment_dir = /var/mail/%d/%n/attach
Thus, the mail of user at domain.tld is stored in 
/var/mail/domain.tld/user/, and mail attachments inside this directory, 
for example: /var/mail/domain.tld/user/attach.
When I start copying (as root while in test mode):
#doveadm backup -u "user at domain.tld" sdbox:/backup/domain.tld/user, 
copying passes, but, as I understand it, the attachments are not copied 
(I am judging so far by the volume of the initial storage and the 
received one, it should not differ VERY much, but it differs by 10 
times, it seems that it’s just the total amount of attachments).
Tell me, plz, how to run doveadm backup correctly so that everything is 
copied so that you can subsequently deploy it from the backup?
Thanks.