PropertiesChanged signal not sent unless Subscribe called

[segfault <segfault@xxxxxxxxxx>]

Hi,

I noticed that some functionality of gnome-settings-daemon doesn't work
as expected on some systems. I tracked that down to the fact that
gnome-settings-daemon subscribes to the
org.freedesktop.DBus.Properties.PropertiesChanged signal of
org.freedesktop.systemd1 but doesn't call the
org.freedesktop.systemd1.Manager.Subscribe() method first.

On the issue I created for that [1], there is some confusion about
whether it's expected that this signal is only sent after calling
org.freedesktop.systemd1.Manager.Subscribe(). The argument is that the
PropertiesChanged signal is part of the D-Bus standard and not defined
in the org.freedesktop.systemd1.Manager interface, so it's unexpected
that it's not sent by default.

[1] https://gitlab.gnome.org/GNOME/gnome-settings-daemon/-/issues/790

Could someone clarify if this behavior is expected?

Thanks!