usb2 problems when connecting multiple devices

I've noticed that when more than one device is connected to the onboard usb2 ports,
one device's (signalling? i guess) interferes with the other, knocking it offline.

It's happened in a number of contexts, only with this hardware:

1. where the os is 13-stable
2. recent 14-stable
3. usb2 is keyboard & wireless mouse
4. usb2 is run0: MAC/BBP RT5390 firmware RT3071 wlan0 & plain usb2 cable
    connecting to a UPS.
5. the pi4 has the recommended 3.1A PSU

At the moment, I don't have a spare usb2 hub to plug into a usb2 port.

I've worked around the situation previously, either by using a powered 
usb3 hub into a usb3 port and plugging the usb2 devices into the hub, 
and/or using just one usb2 device in one usb2 port and leaving the 
other on-board usb2 port empty. 

But both usb ports should be usable and not knock each other offline.

How can I capture relevant information for this for a PR?
--