Mac mini not autobooting after "dirty" shutdown

This Mac mini is intended to run as a kiosk, but a daily sleep/wake cycle is proving problematic with some USB devices.

My plan was to use the "dirty" shutdown from the commands line to simulate a UPS shutdown: sudo shutdown -hu now.

The process is to enable the "Start automatically after a power failure" option in Energy Saver prefs, then to execute the dirty shutdown from the command line, and finally, remove power once the power light goes out, but before the 5 minute timeout.

Tests on other minis have been successful, but this 2018 3.6 i3 Mac Mini is not consistently starting up once you supply power. This morning it started automatically when I plugged it into the mains. But later in the day, additional tests did not result in power being enabled without physically pressing the power button.

Running Mojave 10.14.6. Reset PRAM/NVRAM. pmset -g shows autorestart set to 1. The Mac WILL restart when I forcibly cut power to it.

Please note I do NOT have a UPS connected to the system.

What could be causing the inconsistent behaviour?

The first thing I would try is resetting the System Management Controller (SMC), as it's responsible for power management, sleep/wake/hibernation, restarts/shutdowns, and so on.

Resetting the SMC on a 2018 model Mac mini

For your particular Mac mini you'll need to follow these steps:

1. Fully shut down your Mac mini
2. Press and hold the power button for at least 10 seconds
3. Let go of the power button and wait five seconds
4. Switch your Mac mini on again

Now test to see if this has resolved the issue. If not, then try resetting the SMC with the following steps instead:

1. Fully shut down your Mac mini
2. Unplug the Mac mini from its AC power source
3. Wait at least 15 seconds
4. Plug the Mac mini back into its AC power source
5. Wait at least another five seconds
6. Switch your Mac mini on again

Now test again to see if this has resolved the issue.