The bug

Short:

A water or ice block in the air makes the daylight it lets through decrease with every block in negative y-direction, which is normally NOT a feature of daylight.

Long:

While building an ice farm in a mountains biome, I moved the platform to a certain height. To make sure that the ground beneath the platform was not too dark, I decided to create the platform out of (white stained) glass. Since the Block of water and the ice block are (partially) transparent, I thought, the daylight would reach the ground with a light level of about 12 (Ice and water reduce the light level by 2 or 3). Actually the light level on the ground went down to 6 (distance of 9 blocks to the next not water filled glass block in horizontal direction). After some trial and error, I found out, that the daylight (normally not height dependent) was changed to a height dependent normal light of light level 12, which is decreased by 1, every block downwards.

How to reproduce

1. Set the time to daytime:

   /time set noon

2. Run the following commands in an open area without moving:

   /fill ~-3 ~-3 ~-3 ~3 ~12 ~3 stone hollow
   /fill ~-2 ~12 ~-2 ~2 ~12 ~2 ice

   → Skylight decreases with altitude after passing through the ice