dovecot - Sep 2020 - imapc master user & shared mailboxes

Hello,

I am trying to configure dovecot like this:
https://wiki.dovecot.org/SharedMailboxes/ClusterSetup

imapc_host = 127.0.0.1
imapc_master_user = %u
#imapc_user imapc_password = XXXXXXXX

namespace {
  type = shared
  separator = /
  prefix = shared/%%u/
  location = imapc:~/shared/%%u
}

plugin {
  acl_shared_dict = proxy::acl
}

However, contrary to the article claims, imapc master user is set to
the destination username. This makes it impossible to check the ACLs
against the logged user.

If user.b shares mailbox with user.a, accessing /shared/user.b as
user.a a will log in as "user.b*user.b".

Can we tell dovecot to use the currently authenticated user for
imapc_master_user and the remote user for imapc_user, so it logs in as
"user.b*user.a"?

I'm using dovecot 2.3.11.3 (502c39af9).

Thanks!
Toma