I came upon a lighting bug where a large rectangular portion of the surface had block light = 0, despite the in-game time being about midday and the tallest structure nearby being a two story villager plains house. It is more than a visual glitch as can happen occasionally, as hostile mobs had spawned in great numbers and undead mobs were not taking fire damage.
I am unsure as to how this can be recreated, as I simply happened upon it- though I can say that it occurred outside of the spawn chunks area.
I have uploaded a screenshot of the issue below
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