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.
[media]How to reproduce
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,dungeonFly 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.
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Confirmed, even these dark spots can happen on places with torches.
[media]Even you can see a secondary dark spot on the picture: the church has also a dark spot in spite of the torches...
[media]However when I replace torches, these dark spots disappear.
Even monsters spawn on these dark places in spite of the torches!
Yes, still looks fixed to me. Your issue is MC-142134, I think.
I don't think it's MC-142134, because I didn't have any light sources like torches, beacons or lanterns near the black patches.
Fry
this is odd i would mark this a duplicate but the dark patches are circular which is unlike a similar glitch that is being proccessed