I think this picture says it all:

The steps I took to produce this are:

1. Opened a world with a random seed (6526022573367854339)
2. Render distance + simulation distance 12 chunks, Chunk builder threaded, Smooth lighting maximum, Graphics Fancy, Brightness Bright, Mipmap levels 4
3. I flew around a little and discovered this (at -31, 95, -28).

I have not placed nor broken any blocks, this is completely natural. The lighting also is not just a visual bug, F3 says that the dark blocks are all at light level 0, the bright ones are 4-7, meaning mobs can spawn there.