Why is systemd ignoring my power button?

The systemd service systemd-logind is responsible for handling ACPI signals. First make sure the service is running

systemctl status systemd-logind

If this service is inactive due to a failed "condition check", make sure that dbus is installed and running then try to start the service manually: systemctl start systemd-logind

If the service runs and you see a few "Watching system buttons .." messages in systemd-logind's logs, then systemd should be monitoring your power button. You should check that /etc/systemd/logind.conf isn't changing how systemd handles power button signals.

If systemd still doesn't respond to your power button, make sure that acpid isn't installed and isn't interfering with your system's ACPI events.