Error chunk that has problem with light level appeared in survival mode.
Within dark region, light level of sky represents 0 without no ceiling block.
The region is up to y=95 high.
At some edge of it, light level is gradated up to 14, and zombies doesn't get burned in the region.
Placing block will updates light level partially, but it's gradating like block's light level
However, placing at y=80 or above will correct light level on 3x3 chunk.
Reproducibility is confirmed with the same seed(7501060747914640433), but at different locations.
Before I see them, I entered and exited the Nether.
Sorry for bad English, but I hope this information will help developers.
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I will try to find a solution today and will write to you, wait!

Thank you for your report!
We're tracking this issue in MC-214793, so this ticket is being resolved and linked as a duplicate.
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I'm sorry for the duplication due to lack of research...
I will be careful after that.