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

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

?