unplugging USB solves the issue so yes it is a mobo/BIOS issue i think. something i saw in reddit said to enable 'ErP' setting in my BIOS (Gigabyte), but even after i enabled that I still get the problem. i poked around but didnt see any other settings that may relate to usb power, will continue searching. might try updating BIOS.
I have an ASUS and have ErP disabled.
I remember on my older ASUS motherboard, I had that problem with another USB device staying on. It had to do with some kind of Fast Charge USB port option it had for charging mobile devices.
When i powered off, that usb port was still supplying power.
Do you have a similar usb charge port feature?