Seconding this. Of course check the AFC. But note that the AFC designed to be fully open from 30 feet IIRC, so it should not be necessarily be changing at 70 ft.
But Atomic are picky about mouthpieces, and weird "should not be related problems" come from swapping them out. Freeflows (as in the reg always acting as if the AFC is always activated) happen with some mouthpieces, and not with others.
Especially confusing when there is no reason why things should be affected by a mouthpiece switch, but if you look through the Atomic manufacturers forum you will see there are things with mouthpieces.
Since mouthpiece changes can clearly affect flow, its just possible that once a certain air density is reached it feels a certain way.