Quota issue: enforced, but reporting issue

[dovecot at avv.solutions]

Hello everyone,
I face the following challenge:
- dovecot 2.2.13
- userdb: passwd-file
- quota enabled:
-- default 1GB
-- changed for some users thru users file: 
user at domain:passwd::::::userdb_quota_rule=*:storage=15M
- when sending a message to this user while near quota, warnings are 
correctly triggered => *OK*
- when sending a message to this user while over quota, message is 
refused => *OK*
- when checking quota from mail client (eg Thunderbird 45.2), reported 
quota is.. 1GB => *NOK*
- some config:
mail_plugins = quota notify acl
plugin {
   acl_shared_dict = file:/sd/MAIL_IMAP_POP/%d/shared-mailboxes
   quota = maildir:User quota:ns=
   quota_exceeded_message = Storage quota for this account has been 
exceeded, please try again later.
   quota_rule = *:storage=1G
   quota_rule2 = Trash:storage=+10%%
   quota_rule3 = Spam:storage=+20%%
   quota_status_nouser = DUNNO
   quota_status_overquota = 552 5.2.2 Mailbox is full
   quota_status_success = DUNNO
   quota_warning = storage=90%% quota-warning 90 %u %d
   quota_warning2 = storage=75%% quota-warning 75 %u %d
}
protocol imap {
   mail_plugins = quota notify acl imap_quota imap_acl
   service replication-notify-fifo {
     name = aggregator
   }
   service anvil-auth-penalty {
     name = anvil
   }
   service auth-worker {
     name = auth-worker
   }
   service auth-client {
     name = auth
   }
   service config {
     name = config
   }
   service dict {
     name = dict
   }
   service login/proxy-notify {
     name = director
   }
   service dns-client {
     name = dns_client
   }
   service doveadm-server {
     name = doveadm
   }
   service imap {
     name = imap-login
   }
   service imap-urlauth {
     name = imap-urlauth-login
   }
   service imap-urlauth-worker {
     name = imap-urlauth-worker
   }
   service token-login/imap-urlauth {
     name = imap-urlauth
   }
   service login/imap {
     name = imap
   }
   service indexer-worker {
     name = indexer-worker
   }
   service indexer {
     name = indexer
   }
   service ipc {
     name = ipc
   }
   service lmtp {
     name = lmtp
   }
   service log-errors {
     name = log
   }
   service pop3 {
     name = pop3-login
   }
   service login/pop3 {
     name = pop3
   }
   service replicator-doveadm {
     name = replicator
   }
   service login/ssl-params {
     name = ssl-params
   }
   service stats-mail {
     name = stats
   }
}

- 90-quota:
passdb {
   driver = passwd-file
   args = scheme=CRYPT username_format=%u /etc/dovecot/users
}

userdb {
   driver = passwd-file
   args = username_format=%u /etc/dovecot/users

   # Default fields that can be overridden by passwd-file
   #default_fields = quota_rule=*:storage=1G

   # Override fields from passwd-file
   #override_fields = home=/home/virtual/%u
}

Any idea? Already spent hours on this.
Thank you in advance for the help.
Cheers

A.