Dovecot virtual folders usage: seen in Tbird client if subscribed, treated as _real_ folders -- and archived to local/offline storage? How to prevent?

[PGNet Dev]

i run a dovecot v2.3.10 server.

config includes virtual folders,

        dovecot.conf
                mail_plugins = virtual
                mail_location = maildir:/data/mail/%d/%n/Maildir:LAYOUT=fs:UTF-8
                imap_capability = +SPECIAL-USE
                namespace inbox {
                        inbox = yes
                        type = private
                        separator = /
                        prefix =
                        hidden = no
                        list = yes
                        subscriptions = yes

                        ...
                        mailbox virtual/All {
                                special_use = \All
                                auto = no
                                comment = All
                        }
                        mailbox virtual/Flagged {
                                special_use = \Flagged
                                auto = no
                                comment = Flagged
                        }
                        ...
                }

                namespace virtual {
                        prefix = virtual/
                        separator = /
                        location = virtual:/data/mail/virtual:INDEX=MEMORY
                        hidden = no
                        list = yes
                        subscriptions = yes
                }

and

        /data/mail/virtual/All/dovecot-virtual
                *
                -Trash
                -Trash/*
                All

        /data/mail/virtual/Flagged/dovecot-virtual
                *
                Flagged


in Thunderbird client, without subscribing to the virtula folders, per account, 
they're undisplayed/inaccessible.

if I select 'subscriptions',

        virtual
                All     [X]
                Flagged [X]

I see the aggregated/flagged messages.

When I sync for offline usage, TBird copies messages to local storage for 
offline use, as expected

but it ALSO *copies* all the 'virtual' messages to dirs in the local store

        cd ImapMail/user001-maildir
        tree -L 1 -d virtual.sbd/
                virtual.sbd/
                ├── All        <==========
                └── Flagged    <==========

**DUPLICATING** messages stored in BOTH the parent/original dirs, AND a second 
copy in the Virtual dir.

What needs to be done in order to

        see/use the subscribed Virtual folders in TBird
        enable Offline Sync/Storage
&
        PREVENT the duplication and local storage of the 'virtual' mail 
instances

?