Most (maybe all?) CGMs need to be calibrated regularly with finger pricks. In fact in the case of both the Dexcom G5 (haven't known anyone on the G6) and Medtronic Guardian sensors they can occasionally sort of derp out and ask for multiple calibrations in a row for utterly mysterious reasons. I just asked and my partner says the most times she has had to test to get a Guardian sensor back to normal was seven times in two hours.

Of course this is anecdotal, but the point is that your mileage may vary.