This is how the USB standard works. USB has certain power delivery requirements, and any device exceeding this rating in power consumption will fail. When fully booted the max output must be supplied by a compliant host.
The part which is not obvious is that when a host shuts down, that some “much smaller” power must be available…full voltage, but greatly reduced current ability. When a computer sleeps or goes into a low power mode one would typically still be able to “wake” the computer with a mouse movement or a keyboard key press. This is the minimal power delivery purpose, and thus the port is working as it should (changing this is a “customization”, not a “fix”).
There is a thread about achieving what you want to do which you might be interested in:
https://forums.developer.nvidia.com/t/howto-disable-enable-usb-p-orts-on-nx-programmatically/146125/4
(the NX is very similar to the Nano, but would likely require some changes)
Trivia: PCI also does this. Take a look at “wake on LAN” (“WOL”).