This is actually an issue I've had for a long while now, I've seen it to some extent or other on several Macs I've owned over the years, but on the 2014 Mac Mini it's really bad.
The problem is that when I put the computer to sleep, when it wakes the bluetooth radio is (almost) always disabled. The Bluetooth icon in the menu bar is greyed out. I can right-click the icon and select "Enable bluetooth" from the menu to get it back up and running, so it seems like the hardware itself is fine, and that the drivers are working, but it almost always seems to be off by default after a sleep. Does anyone know why this may be, or what to do about it? Like I said, it's not a critical issue, but it's really annoying.
Mac Mini (2014), OSX High Sierra.