`/var/adm/messages` does not exist until enabling `system-log:rsyslog` SMF service

I've seen in several circumstances that the log file /var/adm/messages does not exist on a Helios system by default. Looking at the SMF services, this seems to correlate with svc:/system/system-log:default being the enabled logging SMF service. I've generally been able to "solve" this by swapping that out for the svc:/system/system-log:rsyslog service. At that point, /var/adm/messages appears, and I see log messages as I'd expect.

I don't have an example system where this is currently the case. However, if I undo the "fix", and re-enable system-log:default, I no longer see messages in /var/adm/messages. I'm either unaware of where those messages are directed if the default flavor is enabled, or there may be an issue with the logging configuration being correctly used to emit such messages.

Here is /etc/syslog.conf on sock.eng.oxide.computer, where I've just done the service-swap song and dance:

#
# CDDL HEADER START
#
# The contents of this file are subject to the terms of the
# Common Development and Distribution License, Version 1.0 only
# (the "License").  You may not use this file except in compliance
# with the License.
#
# You can obtain a copy of the license at usr/src/OPENSOLARIS.LICENSE
# or http://www.opensolaris.org/os/licensing.
# See the License for the specific language governing permissions
# and limitations under the License.
#
# When distributing Covered Code, include this CDDL HEADER in each
# file and include the License file at usr/src/OPENSOLARIS.LICENSE.
# If applicable, add the following below this CDDL HEADER, with the
# fields enclosed by brackets "[]" replaced with your own identifying
# information: Portions Copyright [yyyy] [name of copyright owner]
#
# CDDL HEADER END
#
#ident  "%Z%%M% %I%     %E% SMI"        /* SunOS 5.0 */
#
# Copyright (c) 1991-1998 by Sun Microsystems, Inc.
# All rights reserved.
#
# syslog configuration file.
#
# This file is processed by m4 so be careful to quote (`') names
# that match m4 reserved words.  Also, within ifdef's, arguments
# containing commas must be quoted.
#
*.err;kern.notice;auth.notice                   /dev/sysmsg
*.err;kern.debug;daemon.notice;mail.crit        /var/adm/messages

*.alert;kern.err;daemon.err                     operator
*.alert                                         root

*.emerg                                         *

# if a non-loghost machine chooses to have authentication messages
# sent to the loghost machine, un-comment out the following line:
#auth.notice                    ifdef(`LOGHOST', /var/log/authlog, @loghost)

mail.debug                      ifdef(`LOGHOST', /var/log/syslog, @loghost)

#
# non-loghost machines will use the following lines to cause "user"
# log messages to be logged locally.
#
ifdef(`LOGHOST', ,
user.err                                        /dev/sysmsg
user.err                                        /var/adm/messages
user.alert                                      `root, operator'
user.emerg                                      *
)

Start by inspecting the SMF definitions for system-log:default and system-log:rsyslog, then compare their behavior with /etc/syslog.conf. Reproduce the service swap if possible and determine where messages are directed when the default service is enabled. Done means the expected logging destination is identified and /var/adm/messages is correctly handled or the configuration issue is documented.