(So I'm in the latest snapshot at the time. And this is after 1.13 testing in a world because i seem to have the issue.

To the Main Point:
There seems to be this chunk error were the middle turns dark this in between to chunks in the Y axis.
i think this has to do with the 2 new air blocks that were suppose to be added "cave_air" and "void_air". Which are in command for /fill but when generated are not generated in 1.13+

i go inside with NBT Explorer and notice its only has "minecraft:air" when it generates.
of course there is pictures and i think this would get less confusing (link below).
[link title| !https://image.prntscr.com/image/mmbvD1-ES_yzh6enrMdjrg.png! ]
[link title| !https://image.prntscr.com/image/0RSsw0IgR1m6PxN4Tbh8rw.png! ]

You do not notice this when you (the Person) build this. You do notice it after Logging out of the map and coming back in.

The Interesting part:
so something very interesting is when you place a block the whole chunk goes dark (Attachment below).

you Re-log back into the world the block and the chunk are back to normal.

I believe this is a bug. thank you for taking your time and reading. I tested this on 3 different worlds. generated with 1.13 and Latest snapshot.