Ultra USB Power ports causing issues with shutdown

Hi, bit of a weird one.
Just got a new PC with the Asrock X870 Riptide WiFi motherboard.
I use several external harddrives that are connected to a powered USB hub. The issue is as follows:
- if I connect the powered USB hub to one of the 2 ports that are labelled as "Ultra USB Power" adnd then turn off my PC, within a second of turning off all the case fans start spinning again, at max spee. All the other RGB and fans (e.g. CPU or GPU fans) are off, it's just the case fans that start spinning.
- if I connect the powered USB hub to a different usb port, one that is NOT labelled "Ultra USB Power", everything is as expected - I turn the PC off and no fans are spinning, the PC is "truly" off.
I'm sure it's related to the "Ultra USB Power" port because
- if the issue is happening (i.e. powered USB hub is connected to the "Ultra USB Power" port, PC is off, fans are spinning) and I then unplug the powered USB hub, the fans stop spinning. If a moment later I plug the powered USB hub in again, the fans start to spin (while the computer is off the whole time).
- turning the PSU off, and even unplugging the power cable, doesn't stop the fans from spinning if the powered USB hub is plugged in.

What is going on? Is there a setting I can turn on/off to disable this behaviour?