Despite searching far and wide in the BIOS of the motherboard, Asus ROG Crosshair X670E Hero, I could not find a way to turn off the Polymo Lighting that remains on despite the PC being turned off. Here are some images :
I ran into this issue on my Rampage Extreme VI Omega. This is what worked for me. Since my power supply is ErP compatible, I enabled that in my BIOS. Here's the post about how to do it. If that doesn't work, there are some other suggestions in the same thread.
I confirm that by enabling the ErP (S4+S5) the Polymo Lighting actually turns off and stays off. Of course I lose the "charging" function when the PC is off on the extra front USB Type C port powered by the 6 PIN PCI-E cable but it is not a problem.