So I've started using autopilot lately, and I have come across some issues when setting altitudes. Every time I set a flight mode, say follow at 100ft altitude for instance, it shoots up to well over the altitude I set it at. Also, if I switch to manual flight, the altimeter on the phone display will hit 0 when I am still at least 50ft in the air. It then will have a negative altitude for the rest of the descent. I always set the altitude reference to moving operator in the check list, and I always use an Iphone 6S, so it should have a fairly accurate altitude reading. What am I doing wrong here?