Sunlight penetrates through solid blocks sometimes. Can be reset for a few blocks of height by placing and removing a block above the affected area. Screenshots are of an underground area with no torches, one during the day and one at night.
Searched and found similar issues, but none seem identical to this one, so I hope this isn't a duplicate entry.
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This Happens to me alot!
Duplicate of MC-133553.
Thank you, Kraif. Couldn't find that when I searched, for some reason.