dovecot - Sep 2020 - 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

?