username changed gumby at example.com -> gumby

[Tom]

As you can see in this snip of the maillog, the domain part of the username is being stripped away:

Sep  6 23:12:40 vaj dovecot[114637]: auth-worker(114662): Debug: conn unix:auth-worker (pid=114660,uid=97): auth-worker<2>: passwd(gumby at example.com,10.0.0.70,<YlSArw3oaqoKAABG>): Performing userdb lookup
Sep  6 23:12:40 vaj dovecot[114637]: auth-worker(114662): Debug: conn unix:auth-worker (pid=114660,uid=97): auth-worker<2>: passwd(gumby at example.com,10.0.0.70,<YlSArw3oaqoKAABG>): lookup
Sep  6 23:12:40 vaj dovecot[114637]: auth-worker(114662): Debug: conn unix:auth-worker (pid=114660,uid=97): auth-worker<2>: passwd(gumby at example.com,10.0.0.70,<YlSArw3oaqoKAABG>): username changed gumby at example.com -> gumby
Sep  6 23:12:40 vaj dovecot[114637]: auth-worker(114662): Debug: conn unix:auth-worker (pid=114660,uid=97): auth-worker<2>: passwd(gumby,10.0.0.70,<YlSArw3oaqoKAABG>): Finished userdb lookup

This means that my mail_location will fail since it stores mail by user and domain. dovecot -n:

# 2.3.19.1 (9b53102964): /etc/dovecot/dovecot.conf
# OS: Linux 5.19.6-200.fc36.x86_64 x86_64 Fedora release 36 (Thirty Six)
# Hostname: vaj.example.com
auth_debug = yes
auth_default_realm = EXAMPLE.COM
auth_gssapi_hostname = $ALL
auth_krb5_keytab = /etc/dovecot/dovecot-krb5.keytab
auth_mechanisms = gssapi login plain
auth_realms = example.com
auth_username_format = %u
auth_verbose = yes
debug_log_path = /var/log/dovecot/debug
info_log_path = /var/log/dovecot/info
log_path = /var/log/dovecot/error
log_timestamp = "%Y.%m.%d %H:%M:%S "
mail_debug = yes
mail_location = maildir:%h/Maildir/%d
mbox_write_locks = fcntl
namespace inbox {
   inbox = yes
   location =
   mailbox Drafts {
     special_use = \Drafts
   }
   mailbox Junk {
     special_use = \Junk
   }
   mailbox Sent {
     special_use = \Sent
   }
   mailbox "Sent Messages" {
     special_use = \Sent
   }
   mailbox Trash {
     special_use = \Trash
   }
   prefix =
}
passdb {
   driver = pam
}
ssl = no
ssl_cert = </etc/pki/dovecot/certs/dovecot.pem
ssl_cipher_list = PROFILE=SYSTEM
ssl_key = # hidden, use -P to show it
userdb {
   driver = passwd
}
userdb {
   driver = passwd
   override_fields = mail=maildir:%h/Maildir/%d
}
verbose_ssl = yes

How can I get around this issue? I need mailboxes stored by user/domain.