Certain blocks filter sky light through it rather than allowing it to pass directly like in 1.13.2. This creates an issue only when the blocks are high in the air, eventually making the light zero at such a distance.
Affected blocks (that differ from 1.13.2)
Glass panes (normal and stained)
Fences (but not fence gates)
Walls
Iron Bars
To reproduce
Set time to noon with
/time set noon, and remove the Night Vision effect if it is currently activeDig a hole in the ground 5+ blocks deep
Cover the hole with affected blocks
The light level at the bottom of the hole should decrease
A video example is shown here.
Environment
Java 1.8.0_161 64bit
AMD FX-8370 Eight-Core Processor
NVIDIA Geforce GTX 970
Can confirm; it also affects fences, walls and iron bars, which I've added to the description and summary.