Summary:

Block light does not propagate above 256/230 height. The sides of the light-emitting blocks at 256/230 height are dark. As for mobs / players, if a player / mob is standing at 257/231 height on a block that emits light or above air that contains light, the player will glow.

Steps to reproduce:

1. Place the light emitting blocks at a 256/230 height.
2. Place a few more blocks at the same height.
3. Observe how the light spreads to the blocks as well as the player. Experiment)

Observed results:

Block light does not propagate above 256/230 height, and the top side of full blocks becomes dark.
Also, the player will glow only on light emitting blocks and over air containing light (but only at 257/231 heights).

Expected results:

The light will normally propagate beyond the upper border of the world, as the player in survival can get there.

Image: