The position of the AVC inside the tank is about two thirds of the way down, so it cannot let all the air out. The only plausible explanation is that someone plugged the vent because water was leaking out and then over time the air got absorbed, waterlogging the tank. For that to happen, the airmaker would also have to fail or be disabled. Granted, that too happens whet people put a valve cap on the snifter.
The big nut could also be where the replaceable bladder is inserted.