Is there a monitor that doesn't suffer this issue? My Dell's USB hub power is linked to the monitor power. I can't plug a mouse into the monitor if I want to use that mouse to wake up the computer.
My Dell P2715Q display does work properly as a hub when it's asleep.
The P2715Q has all sorts of quirks. I have a pair of them connected to my Windows computer, and when they go to sleep, Windows basically forgets where all the open windows were and disperses them somewhat randomly. Of all the monitors I've used, I've only ever experienced this with the P2715Q.
I also notice that the power output of the USB ports on the monitors is barely enough to charge any recent mobile device.
That's because it's probably supplying the USB spec of 500ma. That's not a fault, it's how it should work. The usb-looking chargers that come with many recent mobile devices are using a bastardized set of "not really standards" to let them pump more juice through the USB port than is allowed.
But some do allow it to go out of spec, and if that happens you'll only be able to get that extra power if you are using a "charging only" cable (which tricks the phone into thinking it's plugged into a power-only USB outlet and it tries to draw more than 500ma).
So with a USB cable (that's not actually a USB cable), plugged into a USB port (that's not actually a USB port) you can get over that, but it's not meant to run that way on several levels.
It's one of the things that USB-C is meant to solve. Let the device and power source negotiate on how much can be sent and received by each (even being able to swap which way power is going). Sadly that means that cables are no longer just a bunch of metal in a wrapper, they need a set resistance or in some cases built-in IC's, and that means that now a shitty cable can cause issues.
Instead of "sleeping", they appear to make the OS think they've been powered down/disconnected instead. When a monitor goes to sleep, I can hear the "device disconnected" chime in Windows, which should not happen.