systemd version the issue has been seen with

v239

Used distribution

Debian sid
Filed as downstream bug report at https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=902971

When I run journalctl, I saw the following messages:

Jul 04 15:35:45 gtx1080ti-unstable dbus-daemon[25]: dbus[25]: Unknown username "systemd-resolve" in message bus configuration file
Jul 04 15:35:45 gtx1080ti-unstable dbus-daemon[25]: dbus[25]: Unknown username "systemd-timesync" in message bus configuration file
Jul 04 15:35:46 gtx1080ti-unstable dbus-daemon[25]: Unknown username "systemd-timesync" in message bus configuration file

The following users are configured in the dbus config files:

/usr/share/dbus-1/system.d/org.freedesktop.network1.conf:
/usr/share/dbus-1/system.d/org.freedesktop.resolve1.conf:
/usr/share/dbus-1/system.d/org.freedesktop.timesync1.conf:

Since systemd-{networkd,resolved,timesyncd} use DynamicUser=yes, the systemd package in Debian no longer allocates a static system user for them.
On the other hand we have static configuration files which reference those users and are e.g. consumed by dbus-daemon or dbus-broker. Quoting @martinpitt from IRC:

"... the intention is that these [users] are ephemeral
and shouldn't have persisting files or other references in the file system
so any dynamic user referenced in a static policy sounds like a bug, or a case where it shouldn't be dynamic"