This really sounds like something that is probably a crossover issue between the Realtek chipsets that power most of these hubs and the way power is handled with no computer plugged in. Once power is on the chipset, the router / switch knows it has a connected device there and will try to send any broadcast packets to it. Then the chipset probably responds with an error message (possibly to the broadcast address) and that amplifies the problem.
From a design standpoint, there are probably two fixes - one, the hub shouldn’t be passing power to the ethernet chipset if no controlling device is present. But two, the chipset should establish communications to a host device BEFORE it starts responding to network packets or advertising its presence on the network in any way. Everything that comes in before that should be silently dropped.
I can’t think of much you can do about it prior to a hardware redesign or possibly a firmware fix other than set the switch to block excessive broadcast traffic. Which should probably already be true on most corporate networks, but not many home ones. Or train your users to unplug the USB-C power, but that isn’t going to be an easy task - these are meant to be docks where you just have to unplug one thing and walk away with it ready to go next time.
I have another brand USB-C hub with ethernet and PD that I want to test this against now, but it is at my office while I am at home on lockdown. So I can’t do it for a little while, but once things open up again…