In your BIOS Disable ErP Ready/Mode option
Try turning off your WIFI and then shutdown. It's the only workaround I found to keep it from rebooting after shutdown.
i have same board, same video card, same Fenvi, same smbios MacPro7.1, same ssdt, except for the CPU a Ryzen 7 3700x, in my BIOS, ErP Disabled, Above 4G Enabled, UEFI mode, SuperIO disabled. Im on latest firmware with OC 0.6.2