Mac mini not autobooting after "dirty" shutdown
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:


Fully shut down your Mac mini
Press and hold the power button for at least 10 seconds
Let go of the power button and wait five seconds
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:


Fully shut down your Mac mini
Unplug the Mac mini from its AC power source
Wait at least 15 seconds
Plug the Mac mini back into its AC power source
Wait at least another five seconds
Switch your Mac mini on again


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