When stone is rotated together in a certain way. It creates a light grey box of pixels. And looks very unnatural. As you can see in the picture below.
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This is because some blocks (including stone) have their texture rotated randomly depending on their location, which was added in 1.8 I think.
Can confirm in 20w49a. I managed to reproduce this using the follow command and just searching for this error.
/fill ~1 ~ ~1 ~15 ~ ~15 stone
As stated above, stone, along with some other blocks, have their textures randomly rotated which can cause this type of issue.
Really? Mojang?