Smooth lighting is not "smooth" on some blocks

Smooth lighting is not smooth on some blocks, here's a affected block list (maybe incomplete):

• Stairs
• Door
• Glass and variants
• Slab
• Bed
• Shulker box
• Item frame
• Powder snow (MCPE-153532)
• Ice (MCPE-153532)

Steps to Reproduce:

1. Enable smooth lightning in video settings if you disabled it.
2. (Optional, but will let you see better) Set your brightness to 0.
3. Go to some dark places, or set time to night.
4. Place the blocks mentioned above, lighter materials (e.g. smooth quartz) are preferred.
5. Place a light source in the middle of the platform.
6. Observe the light.

Observed Results:
Smooth lighting is not smooth on these blocks.

Expected Results:
Smooth lighting should work on these blocks.

Notes:

Picture 1 (BottomLight.png) showed when the light source is at the level of the plaform. 

Picture 2 (BothLight.png) showed when the light source is at the level of the plaform and one block above the platform, notice how the problem almost disappeared except the non-full side of stairs.

Picture 3 (UpperLight.png) showed when the light source is one block above the platform, which results in slight difference with picture 1.