The bug

In some cases, sand blocks updated upon world generation do not cause light updates, resulting in large dark spots.

Note

This issue previously occurred in "regular" worlds with lakes (MC-233661), but lakes were removed in favor of aquifers in 21w40a, rendering the issue technically fixed. However, it appears that this issue can still occur in single biome / superflat worlds when sand blocks update. I am unsure if this issue affects regular worlds.

Like MC-233661, this issue happens more frequently near spawn chunks. It is solved by relogging into the world or simply updating the chunk lighting by placing a block.

To reproduce

Create the following world and go to the following coordinates.

Type: Superflat
Preset: minecraft:bedrock,3*minecraft:stone,102*minecraft:sandstone,8*minecraft:sand;minecraft:ocean
Seed: 5525418353291932611
Coordinates: /execute in minecraft:overworld run tp @s -31.35 57.41 31.89 355.05 78.15

Expected result

There would be no giant dark spots when sand blocks update upon world generation.