tl;dr: If your bluetooth device just suddenly disappear from your PC (i.e. is no longer in the device manager), to fix:
turn off the computer, turn off the PSU and unplug it, replug and turn on the PSU, turn on the PC. Your bluetooth should be discovered by your PC and usable.
I am a person who prefer wireless peripherals over wired ones. Let it be mice, keyboards or headphones. Wires from the headset rubbing against my shoulder or cluttered desk from mouse and keyboard wires make me nuts sometimes.
About a month or so after I got my new PC with Strix z270i in it, the onboard bluetooth just disappear and no matter how hard I tried to fix it (restarting, reinstalling drivers or even reinstalling Windows) nothing works. And when I about to RMA the board, the bluetooth device is back running a again. (I remember a power outage before that.)
And then it happened again... My computer can't find the bluetooth device on the board. This time I quickly remember that power outage that some how fixed my bluetooth. Then I shut down the PC. Turn off the PSU. Unplug and replug it. And turn it back on. BOOM. FIXED!!1! The bluetooth device is discoverable again!
I registered to this forum just to put this here and hope it helps other people who has the same problem. From reviews on Newegg and Amazon, seems like several people have been having the same problem.