In some cases there is a mainboard jumper or BIOS setting to enable/disable this persistent USB power feature.
I just had a look at the BIOS setup... there must be two dozen acronyms and abbreviations I have no idea what they mean, quite possibly one of them had to do with that. There certainly wasn't anything that simply said "Turn off persistent USB Power".

I am not comfortable experimenting with BIOS settings that I don't understand!
FWIW, on startup the screen flashes "AMD Gigabyte Ultra Durable", and the BIOS setup screen is quite pretty and well laid out, even if it is incomprehensible to me.
tanstaafl.