2018 Mac Mini No Sleep With Catalina And Dell U2415 USB Hub
See title. Note that it would pretend to sleep in the sense that the monitor would go dark, but measuring with a Kill-A-Watt indicated it was still using between 9 and 34 watts, averaging about 11. I first grew suspicious because waking up was very, very fast.
After subbing out various things it became clear the issue is the built-in USB hub in my Dell 2415 monitor. It might matter that the hub includes a USB+power that stays on while the monitor is sleeping.
I'm putting this here partly to warn anyone else who notices a weirdly fast wakeup that the computer might not be sleeping, and what the nature of the problem might be. I also suspect there are probably other hubs that cause a similar sleep failure, and would like to hear about it if anyone else sees a similar problem. And especially if someone figures out a fix, since I actually like using my monitor's hub.
Mac mini, macOS 10.15