dovecot - Feb 2019 - permission denied errors with INDEX=MEMORY and trees plugin

I am using the trees[1] plugin to encrypt the mail on my disk. Because I
don't want it to store any metadata in plaintext, I've also set
INDEX=MEMORY in my mail_location configuration.

However, whenever I get new mail I get errors like the following showing up in
the journal:

lmtp(john at octet.space)<21780><QDkrBqTXclwUVQAAQHGRUQ>: Error:
Mailbox INBOX: Saving mail:
read(/var/vmail/octet.space/john/Maildir/tmp/1551030180.M508113P21780.toaster.octet.space)
failed: Permission denied (read reason=header Subject (Cache file is unusable))

It's like it's trying to read the message after it's been delivered,
but
is not able to because it's been encrypted. I seem to be able to get
email just fine despite the error message. This happens both if I
deliver mail using dovecot-lda or lmtp.

Is there any way to fix this or at the very least suppress the error
message? I have included my dovecot configuration below.

[1] https://0xacab.org/riseuplabs/trees


[root at toaster ~]# dovecot --version
2.3.4.1 (f79e8e7e4)
[root at toaster ~]# dovecot -n
# 2.3.4.1 (f79e8e7e4): /etc/dovecot/dovecot.conf
# Pigeonhole version 0.5.4 (60b0f48d)
# OS: Linux 4.20.8-arch1-1-ARCH x86_64  ext4
# Hostname: toaster.octet.space
base_dir = /run/dovecot/
hostname = toaster.octet.space
mail_gid = vmail
mail_home = /var/vmail/%d/%n
mail_location = maildir:/var/vmail/%d/%n/Maildir:UTF-8:INDEX=MEMORY
mail_plugins = " quota trash zlib trees notify replication"
mail_privileged_group = vmail
mail_uid = vmail
mailbox_list_index_very_dirty_syncs = yes
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 spamtest spamtestplus imapflags notify imapsieve
vnd.dovecot.imapsieve
namespace inbox {
  inbox = yes
  location = maildir:/var/vmail/%d/%n/Maildir:UTF-8:INDEX=MEMORY
  mailbox Drafts {
    auto = create
    special_use = \Drafts
  }
  mailbox Junk {
    special_use = \Junk
  }
  mailbox Sent {
    auto = subscribe
    special_use = \Sent
  }
  mailbox "Sent Messages" {
    special_use = \Sent
  }
  mailbox Spam {
    auto = create
    autoexpunge = 30 days
    special_use = \Junk
  }
  mailbox Trash {
    special_use = \Trash
  }
  prefix = 
}
passdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
plugin {
  imapsieve_mailbox1_before =
file:/var/lib/dovecot/imapsieve/before/report-spam.sieve
  imapsieve_mailbox1_causes = COPY FLAG
  imapsieve_mailbox1_name = Spam
  imapsieve_mailbox2_before =
file:/var/lib/dovecot/imapsieve/before/report-ham.sieve
  imapsieve_mailbox2_causes = COPY
  imapsieve_mailbox2_from = Spam
  imapsieve_mailbox2_name = *
  quota = maildir:User quota
  quota_max_mail_size = 100M
  quota_rule = *:storage=15G
  quota_warning = storage=95%% quota-warning 95 %u
  quota_warning2 = storage=80%% quota-warning 80 %u
  quota_warning3 = -storage=100%% quota-warning below %u
  recipient_delimiter = +
  sieve = file:~/sieve;active=~/.dovecot.sieve
  sieve_before = /var/lib/dovecot/sieve/before/
  sieve_extensions = +spamtest +spamtestplus +notify +imapflags
  sieve_filter_socket_dir = sieve-pipe
  sieve_global_extensions = +vnd.dovecot.pipe +vnd.dovecot.environment
+vnd.dovecot.filter
  sieve_pipe_socket_dir = sieve-pipe
  sieve_plugins = sieve_imapsieve sieve_extprograms
  sieve_quota_max_storage = 4M
  sieve_spamtest_max_header = X-Spam-Score: -?[[:digit:]]+\.[[:digit:]]+ /
(-?[[:digit:]]+\.[[:digit:]])
  sieve_spamtest_status_header = X-Spam-Score: (-?[[:digit:]]+\.[[:digit:]]+) /
-?[[:digit:]]+\.[[:digit:]]
  sieve_spamtest_status_type = score
  trash = /etc/dovecot/dovecot-trash.conf.ext
  zlib_save = xz
  zlib_save_level = 6
}
protocols = imap lmtp
service auth-worker {
  group = smtpd
  user = $default_internal_user
}
service auth {
  unix_listener auth-userdb {
    group = vmail
    mode = 0660
    user = vmail
  }
}
service dovecot-lda {
  executable = script /usr/lib/dovecot/dovecot-lda.sh
  unix_listener sieve-pipe/dovecot-lda {
    mode = 0600
    user = vmail
  }
  user = vmail
}
service filter_rspamc {
  executable = script /usr/lib/dovecot/filter_rspamc.sh
  group = vmail
  unix_listener sieve-pipe/filter_rspamc {
    mode = 0600
    user = vmail
  }
  user = vmail
}
service imap-login {
  inet_listener imap {
    port = 0
  }
  inet_listener imaps {
    port = 993
    ssl = yes
  }
}
service learn_ham {
  executable = script /usr/lib/dovecot/learn_ham.sh
  group = vmail
  unix_listener sieve-pipe/learn_ham {
    mode = 0600
    user = vmail
  }
  user = vmail
}
service learn_spam {
  executable = script /usr/lib/dovecot/learn_spam.sh
  group = vmail
  unix_listener sieve-pipe/learn_spam {
    mode = 0600
    user = vmail
  }
  user = vmail
}
service lmtp {
  unix_listener lmtp {
    group = vmail
    mode = 0660
    user = vmail
  }
}
service pop3-login {
  inet_listener pop3 {
    port = 0
  }
  inet_listener pop3s {
    port = 0
  }
}
service quota-warning {
  executable = script /usr/lib/dovecot/quota-warning.sh
  unix_listener quota-warning {
    group = vmail
    mode = 0660
    user = vmail
  }
  user = vmail
}
ssl = required
ssl_cert = </etc/acme/fullchain.cer
ssl_cipher_list =
TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SHA256:TLS_AES_256_GCM_SHA384:ECDHE-ECDSA-CHACHA20-POLY1305:ECDHE-RSA-CHACHA20-POLY1305:ECDHE-ECDSA-AES256-GCM-SHA384:ECDHE-RSA-AES256-GCM-SHA384:ECDHE-ECDSA-AES128-GCM-SHA256:ECDHE-RSA-AES128-GCM-SHA256
ssl_key = # hidden, use -P to show it
ssl_min_protocol = TLSv1.2
ssl_prefer_server_ciphers = yes
userdb {
  driver = prefetch
}
userdb {
  args = /etc/dovecot/dovecot-sql.conf.ext
  driver = sql
}
protocol lda {
  mail_plugins = " quota trash zlib trees notify replication sieve
zlib"
}
protocol imap {
  mail_plugins = " quota trash zlib trees notify replication imap_quota
imap_sieve imap_zlib zlib"
}
protocol lmtp {
  mail_plugins = " quota trash zlib trees notify replication sieve
zlib"
}