INBOX sharing does not work

[Torsten]

Hello!

INBOX sharing does not seem to work, although all other mailboxes work.

User bar at example.de has shared a few mailboxes:
   # doveadm acl get -u bar at example.de INBOX
   ID                  Global Rights 

   user=foo at example.de        lookup read write-seen 

   # doveadm acl get -u bar at example.de Sent
   ID                  Global Rights 

   user=foo at example.de        lookup read write-seen 

   # doveadm acl get -u bar at example.de Drafts
   ID                  Global Rights 

   user=foo at example.de        lookup read write-seen

But user foo at example.de can't see the shared INBOX:
   # doveadm mailbox list -u foo at example.de
   Trash
   Sent
   Drafts
   Junk
   shared
   shared/bar at example.de
   shared/bar at example.de/Sent
   shared/bar at example.de/Drafts
   INBOX

A review of the ACL does not explain the behaviour:
   # doveadm acl debug -u foo at example.de shared/bar at example.de/INBOX
   doveadm(foo at example.de): Info: Mailbox 'INBOX' is in namespace 
'shared/bar at example.de/'
   doveadm(foo at example.de): Info: Mailbox path: 
/srv/mail-storage/mailhome/example.de/bar/Maildir
   doveadm(foo at example.de): Info: Per-user private flags in mailbox: \Seen
   doveadm(foo at example.de): Info: User foo at example.de has rights: lookup 
read write-seen
   doveadm(foo at example.de): Info: Mailbox found from dovecot-acl-list
   doveadm(foo at example.de): Info: User bar at example.de found from ACL 
shared dict
   doveadm(foo at example.de): Info: Mailbox shared/bar at example.de/INBOX is 
visible in LIST


----------  doveconf -n  ----------
# 2.3.4.1 (f79e8e7e4): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.5.4 ()
# OS: Linux 4.19.0-10-amd64 x86_64 Debian 10.5
# Hostname: XXX.YYYY.de
auth_master_user_separator = *
auth_verbose = yes
first_valid_gid = 401
first_valid_uid = 401
last_valid_gid = 401
last_valid_uid = 401
mail_gid = vmail
mail_home = /srv/mail-storage/mailhome/%d/%n
mail_location = maildir:~/Maildir
mail_plugins = " acl listescape zlib"
mail_uid = vmail
managesieve_notify_capability = mailto
managesieve_sieve_capability = fileinto reject envelope 
encoded-character vacation subaddress comparator-i;ascii-numeric 
relational regex imap4flags copy include variables body enotify 
environment mailbox date index ihave duplicate mime foreverypart extracttext
namespace inbox {
   inbox = yes
   location =
   mailbox Drafts {
     auto = subscribe
     special_use = \Drafts
   }
   mailbox Junk {
     auto = create
     special_use = \Junk
   }
   mailbox Sent {
     auto = subscribe
     special_use = \Sent
   }
   mailbox "Sent Messages" {
     auto = no
     special_use = \Sent
   }
   mailbox Trash {
     auto = subscribe
     special_use = \Trash
   }
   prefix =
   separator = /
   type = private
}
namespace shared {
   list = children
   location = 
maildir:/srv/mail-storage/mailhome/%%d/%%n/Maildir:INDEXPVT=~/Maildir/shared/%%d/%%n
   prefix = shared/%%u/
   separator = /
   subscriptions = yes
   type = shared
}
passdb {
   args = /etc/dovecot/master-users
   driver = passwd-file
   master = yes
   pass = yes
}
passdb {
   args = /etc/dovecot/dovecot-ldap.conf.ext
   driver = ldap
}
plugin {
   acl = vfile
   acl_shared_dict = file:/srv/mail-storage/mailhome/shared-mailboxes.db
   sieve = file:~/sieve;active=~/.dovecot.sieve
   zlib_save = gz
   zlib_save_level = 6
}
protocols = " imap lmtp sieve"
service auth {
   unix_listener auth-userdb {
     mode = 0777
   }
}
ssl = required
ssl_cert = </var/lib/dehydrated/certs/XXX.YYYY.de/fullchain.pem
ssl_dh = # hidden, use -P to show it
ssl_key = # hidden, use -P to show it
userdb {
   driver = static
}
userdb {
   args = /etc/dovecot/dovecot-ldap.conf.ext
   driver = ldap
}
protocol lmtp {
   mail_plugins = " acl listescape zlib sieve"
}
protocol imap {
   mail_max_userip_connections = 20
   mail_plugins = " acl listescape zlib imap_acl"
}
------------------------------

I don't think it is relevant, but I don't want to forget to mention that 
/srv/mail-storage/ is an NFS share.

It would bee nice if someone has a hint. I hope it isn't a bug.

Regards Torsten