Lighting transition is not smooth in darker areas.

I were building a room and noticed that the dark spots (I assume light level 0 or spots that's supposed to be completely dark) were having some strange lighting issues in that the lighting doesn't seamlessly transition from light to dark, as if smooth lighting is turned off. I tried resetting smooth lighting but it didn't work. 

I also noticed that the light flickers sometimes when I place a block, and it's more apparent when the lighting is dim.