I'm building a tall pyramid which extends to Y level 256, and I noticed that many areas below in the general area are dark even with no blocks directly above that area. I can correct the issue by towering up inside the dark spot until it gets bright again, which then corrects some or all of the darkness below.
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No stairs were used in this structure, not sure if that matters. It seemed to make entire chunks dark by making a fake layer of blocks way above at around Y level 220. Placing blocks and then removing them at the affected level got rid of the glitch.
Please provide some additional screenshots or videos of this issue occurring whilst the F3 debug screen is enabled.
I already manually fixed the issues but I will if I run across it again.

Might relate to / duplicate MC-188525.
Yes that looks like the same issue. Also, now it is saying a sky light level of 15 inside my pyramid where there is no sky access.
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Could be tangentially related to MC-170010 also? Looks a bit similar visually. I am not sure.

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I suspect this is a case of MC-43968.