The Bug:

Smooth lighting renders incorrectly at the edges of spaces that are one block tall or one block wide.

Steps to Reproduce:

1. Ensure that you have smooth lighting enabled.
2. Summon a one block tall space of stone blocks with some light inside by using the commands provided below.

   /fill ~1 ~ ~1 ~15 ~2 ~15 minecraft:stone hollow

   /setblock ~8 ~1 ~8 minecraft:light

3. Switch to spectator mode and look at the lighting toward the east side of the one block tall space.

Observed Behavior:

Smooth lighting renders incorrectly thus creating harsh transition between light levels.

Expected Behavior:

Smooth lighting would render correctly and smoothly.