Today I was mining an entire Chunk just for fun, my shulkerboxes were full, so I had to go to my base, when I returned, half of the chunk's walls weren't visble, but the floor was. There were some caves nearby (I covered them with stone so my hole would look better), but those couldn't be seen with this bug, unlike X-ray glitches, the mobs in the caves were visble though. When I jumped to the bottom of my hole at y=23 (I still hadn't finished mining), everything was back to normal.
As far as I can tell, it was just a visual glitch, since I couldn't go trough those walls and nothing special happened.

Notes:
1.- I was mining exactly one chunk, so the walls of the hole were the edge of the chunk (that might have caused the error, or at least contributed to it.) Also, this visual glitch only happens when you stand on the outer edges of the chunk, and not other parts, for example, in the middle of the chunk

2.-I was playing on the beta, but was mining a chunk generated before, where bedrock was at layer 0 (instead of -64 in the newer chunks)

3.- My simulation distance was 4 chunks, but my render distance was and always has been 15 chunks.