Falling off Lower Ascent.

The auto-teleport is a little box at the bottom of the map. Every map has a built in "return to map" function that puts you back on the map if you fall off. If you fall past the auto-teleport without being affected by it, you'll be returned via the built-in method rather than the auto method.

Some maps don't have the backup method you described (via falling through to the bottom of a map you're not suppose to fall down in, such as Kerning Swamp). I afked literally for hours at the very bottom of one of Eos Tower's floors, and I came back later and I was still there.