Fairy lights of a certain shorter length do not produce any in-world light. A comparison between 2 short fairy lights and a slightly longer fairy light can be seen below. The latter is the only one seen to produce actual light.

Mod version: 2.1.2-1.12.x
Forge version: 1.12.2-14.23.4.2744

I'm having the same issue. No optifine installed either. I can't place a single light cause it won't light up. :(