I also have this similar issue, and what I've done that temporarily fixes it is a PRAM reset. However, it goes away as soon as you boot back into Mojave. This is the case for the Magic Keyboard, as well as other bluetooth peripherals connected to the laptop.
For reference, I'm using a MacBook Pro 15" 2017, running Windows 10 LTSB.
I'm also looking for a more permanent solution, and I searched in the past threads and tried it out to no avail. In my case, I believe the problem comes from not being able to update the bluetooth driver. When looking in device manager, the bluetooth driver simply says "bluetooth radio", and does not update itself when I tried to install the Broadcom bluetooth file from the Boot Camp Windows support software. I also tried selecting the driver manually in the driver settings, but that did not work either.
Another thing to note is that I did not have this problem prior to updating to Mojave, it worked fine in High Sierra.
Thanks!