I think this picture says it all:
[media]
The steps I took to produce this are:
Opened a world with a random seed (6526022573367854339)
Render distance + simulation distance 12 chunks, Chunk builder threaded, Smooth lighting maximum, Graphics Fancy, Brightness Bright, Mipmap levels 4
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.
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