There are many things to try to exclude possible causes.Would you know how I can troubleshoot this a little deeper before I try and report it?
Power-off still doesn't work without anything except / mounted? This means it's not a storage device having trouble.
What happens after the physical shutdown button? In case no power-off, the PSU doesn't get a signal from the operating system. --> Is the ACPI module present and working correctly? But is NUC ATX anyway? It could be different.
Many more things...