Mmmm… I heard something about this sorta thing happening before, where your blood glucose readings are higher even though you haven’t eaten. It’s something to do with your liver releasing its surplus of stored sugar into your blood stream, which is actually supposed to be a good thing. Helps cut down on the fatty liver aspect.
I don’t know how overly concerned I’d be with high blood glucose levels while doing a fast (unless it somehow spiked into dangerously high ranges, but 86 is still well within the normal range). Probably better to monitor for low blood glucose.