I loaded a 18w16a world in 18w20a, and noticed that the lighting is wrong. I have sky light level 15 everywhere, even deep underground. When I noticed that, I built a 4x4 test room with a roof and no light source (should be totally dark), but I had Light 14: sky 14, block 0.
But this depends on the location, when I built such a test room at another location, it was totally dark (Light 0: sky 0, block 0) as expected.
Sky light level 15 everywhere has advantages and disadvantages. First I noticed that there spawned an enormous amount of slimes in my slime farm (because the whole perimeter was lighted at daytime I guess), that was fine, but then I noticed that my fully automatic carrot and potatoe farms didn't work anymore, because the uprooting of the center plant with light level 7 doesn't work anymore and the farming villager gets into idle mode.
Related issues
Comments

This may be a different issue from the one that it "duplicates" as this is a very reproducable bug - update a world from 16a to 20a and it is obvious that it occurs. On the other hand, the bug that it supposedly duplicates is rather inconsistent and not readily reproducable, suggesting that it may stem from a different issue in code.