Basically same problem from vanoudt/KOHAExtra
Tried all their steps nothing. Checked journalctl nothing that I could see.
sudo shutdown, systemctl poweroff all have the same effect, the computer turn off (rgb and fans spin down) then a second later it starts powering on again.
I have old Nvidia card that failed to shutdown if using Nouveau driver. Just want to make sure, did you already have Nvidia driver installed? Check with lsmod | grep -iE "drm|nvidia|nouveau"