For some reason at certain altitudes above ground level (seems to mostly depend on ground level, or rather distance from the naturally generated terrain), light levels start acting weirdly.
The light levels start decreasing mostly by decrements of 1, but can vary.

With some testing I have come to a conclusion that it doesn't purely depend on distance from the ground but it is greatly influenced by it.

Also some odd things about this is blocks seem to cast a shadow next to them (with testing, seems to be directional, in my case its a decrement of 1 towards south and east, and a decrement of 2 towards north and east)

There is plenty more strange behaviour with light at altitudes, but I cannot find pattern or logic to it with simple testing while placing blocks and looking at the f3 screen

EDIT:
Reloading world (as MC-236633 suggests, as a way to fix the problem) doesn't really fix it as the bug just now thinks of the previously placed blocks as the "ground blocks" mentioned previously, and now the bug still appears a certain distance from those blocks.

After further investigation with the new problem, it seems that the bug isn't only vertical but happens horizontally from the blocks placed before reloading the world.