I first noticed the issue in Pre-Release 2, but it might have been present in Pre-Release 1 already, certainly absent in 19w14b though: quite a few areas of the world have developed patches of darkness, sometimes visible on ground level, sometimes starting higher up, but invariably ending beyond the build height, often around y 275 (so 20 blocks above build height). In addition, some chunks have been corrupted and regenerated.