During the day, light will not spread beneath trees or under certain other things. Within the chunk it will be extremely dark under an oak tree (for example, I've also seen it under birch trees) - sometimes almost completely black. This will happen for multiple trees, inside doorways of a building, and occasionally under lower half slab roofs
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We're tracking this issue in MCPE-58182, so this ticket is being resolved and linked as a duplicate.
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