Sky light level is 15 everywhere, even deep underground

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.