Steps to reproduce

1. Make a large, fully dark area.
2. Place two light sources such that they create two spots with light level 1 next to each other.
3. Place a solid block in a location that reduces one of the light level spots to 0.

Expected result

The other light level 1 spot remains light level 1.

Observed result

The other light level 1 spot changes to light level 0, allowing monsters to spawn.

Alternate steps

1. Load MCPE-151266.mcworld
2. Press the "Reset light" button.
3. Press the "Place blocks" button.
4. Watch the north side of the snow for a few minutes.

Expected results

Nothing spawns north of the snow because the torches to the north are 14 blocks away, so the block light just north of the snow should be at level 1, and it is at level 1 before the snow is placed.

Observed results

Monsters spawn on the blocks north of the snow, where block light should be 1 but is miscalculated to be 0 after the snow blocks are placed. See MCPE-151266 bad spawn.mp4