I do not yet know the conditions necessary to reproduce this, as I tried to on creative using various restrictions (using netherrack or not, in the Nether or not, etc) but was not able to reproduce it outside of the original world where I found it, where I can still reproduce the bug reliably. But anyway.

It seems that it is possible to clip into a one-block-high opening in a wall, as seen in the video below. I ran into this problem while making a tunnel near the Nether ceiling in a Crimson Forest biome, although again I'm not sure if this has anything to do with it. I happened to run into a very tall cave, so the bottom row of wall blocks in my tunnel were open. Facing the wall, I walked towards it and clipped under the block, going into crawl mode, and I was able to fall out the other side.