The area around the glowstone looks like the typical shadow bug, that is solved by putting a nearby block to update the error, but not, since once solved... after time the same shadow reappears in the same place.
Edit:
That at height 64 or higher the light level is 0 even if it stays next to the glowstone, as you can see in the second image.
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However, this issue has been resolved as a Duplicate of MC-116690.
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