The bug

Starting with snapshot 19w02a, there are pitch black patches and dark spots quite commonly in the world, mostly in the plains biome. I haven't seen these in any other 1.14 snapshot that includes the new lighting system introduced in 18w43a.
I could imagine that some of the new code has been reverted to improve performance.

How to reproduce

1. Create a superflat world with the Overworld preset:

   minecraft:bedrock,59*minecraft:stone,3*minecraft:dirt,minecraft:grass_block;minecraft:plains;village,biome_1,decoration,stronghold,mineshaft,lake,lava_lake,dungeon
   

2. Fly around
   → You will find lakes with black patches and dark spots in a minute or two

Addendum

Now that I dug deeper into it, I think this is just a reappearance of MC-29490 which got fixed in 18w06a when the new world generator has been introduced.