Disc brake lost power
I also had noise and power issues with my m8000s when they were new.  Sanding the pads and cleaning the rotors made no difference.  My solution was to remove the pads, clamp them in a vise, and hit them with a propane torch for about 30 secs each.  They briefly flamed up, which to me indicated contamination.  Once reinstalled there was power and silence.  That was over a year ago and the problem has not returned.

There is a good thread on MTBR about this issue.

http://forums.mtbr.com/brake-time/xt-xtr-brakes-get-squeal-power-loss-after-sitting-idle-6-8-weeks-902792.html