When you have no blocks above you, and your Y coordinate is greater than zero, doing /asc will not change your location. But when your Y coordinate is less than zero, it will change it to Y=256. This mostly only affects Aether kit players who can do this in the End, but there are a few locations in the Overland where you could see it as well. I discovered it while in Lightdonia, when a region did not load, and confirmed the same behavior in the End dimension.