I've researched on this topic and tried all of the recommendations such as editing kernel options acpi=off, acpi=noirq, acpi=force, then sudo update-grub. I also disabled wake on lan in the BIOS. Unfortunately, nothing works.
/(root) is mounted to a 256 SSD/home is mounted to a 1TB HDD
After reading this kernel bug report I found out that BIOS settings cause this behaviour. It says there that you should enable keyboard and mouse power on. Now the reboot on shutdown is fixed.
You have forgot to shut down one or more programs that you had running. I have the same problem with you and i find out that the Psensor was open when i was trying to shut down my pc. When i shut down Psensor the problem was solved.
