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 active
• Dig 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.