I have had this issue since I bought the board a few years ago - when I set the option to allow PCI-E devices to wake the machine, in order for WoL to work, WoL does seem to work but the machine will not stay powered off. After something like 30 seconds it will wake again even if the network cable is unplugged.
I think this my be happening due in some way to the monitors and graphics card (ASUS Strix 2080 Super) but a quick try of powering off the monitors before powering off the device via SSH found it still powered on after a short interval.
Has anyone seen this before? Any ideas?

Is the power switch, by chance, set to do a suspend instead of power off? If so it may not be getting a full suspend but something is actually preventing the suspend.

For some bios versions the action of the power switch may be set within the bios. For others it can be set within the OS, and those settings may not be compatible.

For example, within windows the setting of the power switch is user configured. Within gnome on my system the 'settings --> power' panel has a setting choice for the power button of 'power off', 'suspend', or 'nothing'.

I'm not powering off via the power switch, I either use the option in XFCE to power off or run "shutdown -h now" in the terminal locally or through SSH in the tests with the monitors powered off.
The power switch is set to the usual "press for power menu, press and hold to, power down".
The machine is definitely not suspended as I see the full boot sequence every time it powers on.

I'll mark this as solved, hopefully not prematurely, as I just installed the latest BIOS and WoL now seems to just work without any of the weirdness. The BIOS I installed is 6063 from earlier this year so I am not sure when it was fixed. Good to know they're still working on the BIOS though.